Improved pressure control in an exhaust aftertreatment system

Abstract

The invention relates to methods for regulating the pressure in a pressurised supply line for supplying reactant for selective catalytic reduction to an exhaust gas stream in an exhaust gas aftertreatment system, the exhaust gas aftertreatment system comprising: - a feed pump for conveying the reactant, - the supply line for supplying the reactant to the exhaust gas stream, wherein the supply line is fluidly connected to the feed pump, - a pressure regulator designed to adjust the delivery rate of the feed pump as a function of the pressure of the reactant in the feed line by means of a control having at least an integral component, the method comprising the following steps: - Detecting the pressure of the reactant in the supply line, - Detecting whether a pressure curve of the reactant pressure in the supply line, which is characteristic of the occurrence of air bubbles in the reactant, occurs, - Performing a reset of the integral component depending on the detection of a pressure curve characteristic of the occurrence of air bubbles.

Description

Technical field

[0001] The invention relates to a method for regulating the pressure in a pressurized supply line for supplying reactant for selective catalytic reduction. The invention also relates to a pressure regulator for regulating the pressure in a pressurized supply line for supplying reactant. Furthermore, the invention relates to an exhaust gas aftertreatment system for exhaust gas aftertreatment in an exhaust gas stream by means of selective catalytic reduction. The invention also relates to an exhaust system of a motor vehicle comprising an exhaust gas aftertreatment system, and to a motor vehicle comprising an exhaust system with an exhaust gas aftertreatment system. Technical background

[0002] In exhaust gas aftertreatment systems, the proportion of nitrogen oxides contained in the exhaust stream is reduced by means of selective catalytic reduction (SCR). Such exhaust gas aftertreatment systems may, for example, include a feed system that supplies the reactant required for the catalytic conversion to the exhaust stream. It has been found that the currently used reactant pressure control system results in pressure spikes caused by air ingress.

[0003] It is therefore an object of the invention to provide an improved control of the pressure under which the reactant is subjected, in which such pressure peaks are avoided. Disclosure of the invention

[0004] The invention relates to methods for regulating the pressure in a pressurised supply line for supplying reactant for selective catalytic reduction to an exhaust gas stream in an exhaust gas aftertreatment system, the exhaust gas aftertreatment system comprising: - a feed pump for conveying the reactant, - the supply line for supplying the reactant to the exhaust gas stream, wherein the supply line is fluidly connected to the feed pump, - a pressure regulator designed to adjust the delivery rate of the feed pump as a function of the pressure of the reactant in the feed line by means of a control having at least an integral component, the method comprising the following steps: - Detecting the pressure of the reactant in the supply line, - Detecting whether a pressure curve of the reactant pressure in the supply line, which is characteristic of the occurrence of air bubbles in the reactant, occurs, - Performing a reset of the integral component depending on the detection of a pressure curve characteristic of the occurrence of air bubbles.

[0005] The pressure increase observed in prior art solutions when air bubbles appear can be caused in particular by the integral component of the control. The solution according to the embodiments of the invention therefore proposes detecting the pressure curve of the reactant pressure characteristic of the occurrence of air bubbles and resetting the integral component of the control whenever this characteristic pressure curve occurs. In this way, for example, it can be achieved that the pressure regulator can react more quickly than with previous solutions to the pressure increase associated with the occurrence of air bubbles. In particular, it can be achieved, for example, that the pressure is adjusted back to the setpoint by the pressure regulator within a short period of time.

[0006] This makes it possible, in particular, to avoid pressure peaks of the reactant in the reactant feed line. In particular, this can also prevent, for example, a complete or partial shutdown of the exhaust gas aftertreatment system as a result of pressure peaks. The reactant can be a reaction solution, for example. By avoiding pressure peaks, the hydraulic load on the components of the exhaust gas aftertreatment system can be reduced. This can, for example, extend the service life of the components of the exhaust gas aftertreatment system, in particular the conveying device and the dosing device. Furthermore, improved dosing accuracy can be achieved, for example.

[0007] The invention also relates to a pressure regulator for regulating the pressure in a pressurized supply line for supplying reactant for selective catalytic reduction to an exhaust gas stream in an exhaust gas aftertreatment system, wherein the exhaust gas aftertreatment system comprises a feed pump for feeding the reactant, which is fluidically connected to the supply line, wherein the pressure regulator is designed to - to regulate the delivery rate of the feed pump as a function of the pressure of the reactant in the feed line by means of a control system which has at least an integral component, - to detect a pressure curve of the pressure of the reactant in the supply line which is characteristic of the occurrence of air bubbles in the reactant, and - to reset the integral component depending on the detection of a pressure curve characteristic of the occurrence of air bubbles.

[0008] Furthermore, the invention relates to an exhaust gas aftertreatment system for exhaust gas aftertreatment in an exhaust gas stream by means of selective catalytic reduction, the exhaust gas aftertreatment system comprising: - a feed pump for conveying the reactant, - a supply line for supplying the reactant to the exhaust gas stream, wherein the supply line is fluidly connected to the feed pump, - a pressure regulator designed to regulate the delivery rate of the feed pump as a function of the pressure of the reactant in the feed line by means of a control having at least an integral component, wherein the pressure regulator is designed - to detect a pressure curve of the pressure of the reactant in the supply line which is characteristic of the occurrence of air bubbles in the reactant, and - to reset the integral component depending on the detection of a pressure curve characteristic of the occurrence of air bubbles.

[0009] Furthermore, the invention relates to an exhaust system of a motor vehicle which comprises an exhaust aftertreatment system as described above.

[0010] The invention also relates to a motor vehicle comprising an exhaust system with an exhaust aftertreatment system as described above.

[0011] According to a preferred embodiment, the control has a proportional component and an integral component.

[0012] Preferably, the pressure regulator is designed to regulate the pressure of the reactant in the supply line to a desired pressure value.

[0013] It is advantageous if the pressure regulator is designed to regulate a speed of the feed pump depending on the pressure of the reactant in the supply line.

[0014] According to a preferred embodiment, detecting the pressure curve characteristic of the occurrence of air bubbles comprises detecting whether a lower threshold value of the pressure is undershot, in particular for a predetermined period of time. If air bubbles enter the exhaust gas aftertreatment system, a pressure drop may initially occur, for example. This pressure drop can be detected, for example, using the lower threshold value.

[0015] According to a further preferred embodiment, detecting the pressure curve characteristic of the occurrence of air bubbles comprises detecting whether the pressure first falls below a lower threshold value for a predetermined period of time and later exceeds an upper threshold value. The occurrence of air bubbles in the exhaust gas aftertreatment system is typically characterized by an initial pressure drop followed by a subsequent pressure increase. This pressure increase can be detected, for example, using the upper threshold value.

[0016] It is advantageous to limit the number of consecutive resets of the integral component. This can, for example, prevent a continuous reset loop.

[0017] Preferably, the number of resets of the integral component carried out is counted by means of a counter, wherein, in the event that the number of resets carried out exceeds a predetermined maximum value, no further reset of the integral component is carried out for at least a predetermined second period of time.

[0018] According to a preferred embodiment, if the number of resets performed exceeds a predetermined maximum value, a time interval is started, wherein after the expiration of the time interval the counter for the number of resets performed is reset.

[0019] Preferably, the exhaust aftertreatment system comprises a tank for the reactant. Brief description of the drawings

[0020] Embodiments are explained in more detail below with reference to the attached drawings. They show: Fig. 1 an exhaust aftertreatment system; Fig.2 a more detailed representation of the exhaust aftertreatment system; Fig. 3 the pressure of the reactant and the speed of the feed pump unit as a function of time in a prior art solution; Fig. 4 a flowchart of a method for pressure control; Fig. 5 the pressure of the reactant and the speed of the feed pump unit as a function of time when using the Fig. 4 shown procedure. Description of embodiments

[0021] Fig.Figure 1 shows an exhaust gas aftertreatment system, in particular for a diesel engine, which is particularly designed to reduce the proportion of nitrogen oxides in the engine's exhaust gases by means of selective catalytic reduction (SCR). To carry out the selective catalytic reduction, a reactant is added to the exhaust gas stream, in particular a reaction solution, for example an aqueous urea solution known as "AdBlue."

[0022] The Fig.The exhaust gas aftertreatment system shown in Figure 1 comprises an exhaust line 2 and a catalyst 4, with an exhaust gas stream 6 flowing through the exhaust line 2 and the catalyst 4. The catalyst 4 is designed to catalyze the reaction of the nitrogen oxides contained in the exhaust gas stream 6 with the supplied reactant, resulting in nitrogen and water. The temperature and nitrogen oxide content of the exhaust gases are monitored by means of a first temperature sensor 8, a second temperature sensor 10, and a NOx sensor 12.

[0023] A dosing unit 14 is provided on the exhaust line 2 for supplying the reactant to the exhaust gas stream 6. The dosing unit 14 can, for example, be designed to spray the reactant into the exhaust gas stream 6. A conveying device 16 is fluidically connected to the dosing unit 14 via a supply line 18. The conveying device 16 is designed to convey the reactant via the supply line 18 to the dosing unit 14. The conveying rate is regulated such that the pressure in the supply line 18 is adjusted to a predetermined target value, for example, to 9 bar. The conveying device 16 is fluidically connected via an intake line 20 and a return line 22 to a tank 24 containing a volume 26 of reactant.

[0024] The reactant contained in tank 24 is fed to the conveying device 16 via the intake line 20 and conveyed from the conveying device 16 to the dosing unit 14. Excess reactant returns to tank 24 via the return line 22. A third temperature sensor 28 is arranged in the wall of tank 24 and is designed to measure the temperature of the reactant. A fill level sensor 30 is also mounted on or in tank 24.

[0025] To preheat the volume 26 of reactant contained in the tank 24, coolant heated by the engine is passed through a first coolant line 32, a heating element 34, and a second coolant line 36. The coolant is then passed through the conveyor device 16 via the second coolant line 36 and discharged via the third coolant line 38 to heat the reactant within the conveyor device 16.

[0026] The dosing unit 14, which is heated considerably by the passing exhaust gas stream 6, is cooled by the engine coolant. To cool the dosing unit 14, the engine coolant is routed via the coolant supply line 40, the dosing unit 14, and the coolant return line 42.

[0027] A dosing control unit 44 can optionally be provided to control the dosing unit 14. The dosing control unit 44 is designed to transmit control signals 46 to at least one actuator, in particular to the dosing unit 14. Furthermore, the dosing control unit 44 is designed to receive and evaluate sensor signals 48 from at least one sensor, for example, from the fill level sensor 30. The dosing control unit 44 can also be connected to an engine CAN bus 50 and / or a diagnostic CAN bus 52.

[0028] In Fig. 2 is the functionality of the Fig.1 shown exhaust aftertreatment system is shown in detail. In Fig. 2 shows the conveying device 16, which comprises a feed pump unit 54 with a pump 56 driven by a pump motor 58. The feed pump unit 54 can optionally be equipped with an inlet valve 60 and an outlet valve 62. During operation, the feed pump unit 54 draws in the reactant stored in the tank 24 via the suction line 20. The drawn-in reactant is conveyed through the switching valve 64, the feed pump unit 54, and again through the switching valve 64, and reaches the dosing unit 14 via a pressure damper 66 and the supply line 18. The dosing unit 14 comprises a dosing valve 68.

[0029] A control unit 70 is provided to control the processes in the exhaust gas aftertreatment system. The control unit 70 includes, among other things, a metering valve control 72 for controlling the metering valve 68. The metering valve control 72 can, for example, be part of the Fig. 1 shown dosing control unit 44.

[0030] A pressure sensor 74 is provided in the conveying device 16 and is designed to detect the pressure of the reactant in the supply line 18. A pressure sensor signal generated by the pressure sensor 74 is fed to the control unit 70, where it is evaluated by a pressure signal evaluation unit 76. The pressure signal evaluation unit 76 is designed to determine the actual value of the pressure in the supply line 18 based on the pressure sensor signal.

[0031] The control unit 70 also includes a pressure regulator 78, which is designed to regulate the delivery rate of the feed pump unit 54 as a function of a predetermined setpoint pressure and the actual pressure value provided by the pressure signal evaluation unit 76, such that the pressure in the supply line 18 approaches the setpoint. The pressure regulator 78 is designed to generate a suitable control signal for the feed pump unit 54 and transmit it to it. For example, the speed of the feed pump unit 54 can be adjusted as a function of the control signal.

[0032] The supply line 18 is fluidically connected to a return line 80. Via the return line 80, a return throttle 82, and an optionally provided shut-off valve 84, the reactant can be returned from the supply line 18 to the tank 24 if necessary.

[0033] The control unit 70 also includes a switching valve control 86 for the switching valve 64. In the Fig. In the position of the switching valve 64 shown in Figure 2, reactant is conveyed from the tank 24 to the supply line 18. By actuating the switching valve 64, the conveying direction can be reversed, for example, to convey reactant from the supply line 18 back to the tank 24.

[0034] In addition, the control unit 70 comprises a heating control 88 for the conveyor device 16, wherein the heating control 88 is designed to keep the conveyor device 16 at a predetermined temperature.

[0035] The pressure regulator 78 will be discussed in more detail below. As already described above, the pressure regulator 78 is designed to regulate the pumping power of the feed pump unit 54 as a function of the setpoint pressure and the actual pressure value so that the pressure in the supply line 18 approaches the setpoint. A proportional-integral controller is preferably used to regulate the pumping power, which has a proportional component and an integral component. Such controllers are also referred to as PI controllers.

[0036] The PI controllers currently in use already provide for a reset of the integral component in certain situations, i.e., for resetting the integral component of the PI controller to zero in certain situations. The state-of-the-art PI controllers currently in use, on the one hand, provide for resetting the integral component whenever the reactant begins to be pumped, for example, when the vehicle is started. Furthermore, the integral component is also provided for resetting whenever a system check is performed to detect hydraulic blockages.

[0037] During operation, it is possible for air to enter the conveying device 16 from the tank 24 via the suction line 20. In this case, the performance of the conveying pump unit 54 is significantly impaired by the air.

[0038] As a result of the air ingress, the pressure in the supply line 18 initially drops. The integral component of the PI controller then sets a higher speed of the feed pump unit 54 (up to a maximum limit) to bring the pressure back to the setpoint. However, as soon as the air has escaped from the feed device 16, the pressure rapidly rises to values ​​above the setpoint, e.g., 9 bar, because the control system cannot react quickly enough to the pressure increase due to the integral component of the control system, and the speed of the feed pump unit 54 cannot be reduced quickly enough.

[0039] Typically, the occurrence of air bubbles in the conveying device 16 initially leads to a reduction in the pressure in the supply line 18, with this pressure drop then being followed by a sudden, strong pressure increase as a result of the behavior of the PI regulator.

[0040] In Fig.Figure 3 shows this sudden pressure increase currently observed in prior art pressure regulators. The pressure curve 90 indicates the pressure of the reactant in the feed line 18 as a function of time, whereas curve 92 shows the speed of the feed pump unit 54 as a function of time. After the air bubbles have left the feed device 16, a pressure increase occurs as shown by arrow 94. The pressure rises to values ​​significantly above the nominal pressure of 9 bar. As soon as the nominal pressure of 9 bar is exceeded, the speed of the feed pump unit 54 is reduced, as shown in Fig.3 is shown by arrow 96. However, due to the integral component of the control, the speed decreases only slowly. Pressure curve 90 shows that the pressure increase during phase 98 is gradually regulated. Subsequently, the pressure slowly drops back to the nominal pressure in the area indicated by arrow 100. However, even in the area indicated by arrow 100, stable pressure conditions are not yet achieved.

[0041] The Fig.The pressure increase shown in Figure 3 causes several problems: As a result of the rapid pressure increase, the pressure in the hydraulic system becomes too high. In general, the pressure peaks lead to greater stress on the entire hydraulic system. If the pressure reaches a pressure above the threshold value of 12.5 bar specified for the components for a period of more than 5 seconds, there is a risk that the system will even shut down. In addition, the pressure peaks may reduce the service life of the components, in particular the service life of the conveyor 16 and the dosing unit 14. Dosing accuracy can also be impaired by the excessive pressure, which can, for example, lead to higher dosing quantities. These disadvantages can lead to customer complaints.

[0042] To avoid these disadvantages, the solution according to the embodiments of the invention provides for detecting the presence of air bubbles in the supply line and for initiating a reset of the integral component of the pressure regulator whenever air is detected. By resetting the integral component to zero, the sharp pressure increase observed in previous prior art solutions, which was (partly) caused by the integral component of the control, can be avoided or reduced. This allows the pressure in the supply line to be adjusted back to the target value in a comparatively short time.

[0043] To detect the occurrence of air bubbles in the supply line 18, it is preferably provided to monitor the pressure and to check whether a pressure curve characteristic of the occurrence of air bubbles occurs. When air enters the supply line 18 via the intake line 20 and the feed pump unit 54, this initially causes a pressure drop. The pressure control reacts to this pressure drop by increasing the delivery capacity of the feed pump unit 54, which subsequently leads to a pressure increase in the supply line 18.

[0044] This characteristic pressure curve can preferably be detected using two threshold values, a lower threshold value and an upper threshold value. When air occurs in the system, the pressure first falls below the lower threshold value and then exceeds the upper threshold value. For reliable detection of an event, the pressure should preferably be below the lower threshold value for at least a predetermined first time period t1, whereby the first time period t1 can be recorded using a timer provided for this purpose. The first time period t1 can, for example, be set to 1 second. In this way, the detection of events is made insensitive to short pressure fluctuations, which can be caused, for example, by a stiff system during long travel times of more than four hours or by high dosing requirements.

[0045] Exceeding the upper threshold then serves as a trigger for resetting the integral part of the control in order to avoid a further sharp increase in pressure.

[0046] The pressure control procedure is shown in the flow chart of Fig.4. The method starts in step 102. In query step 104, a query is made as to whether the current pressure of the reactant in the supply line 18 is below the lower threshold value and thus also below the upper threshold value for at least a first time period t1. If this is not the case, no event has occurred, and the method ends in step 106. If, on the other hand, it is determined in query step 104 that the pressure is below the lower threshold value for at least the first time period t1, then the subsequent query step 108 determines whether there is a subsequent pressure increase to a pressure above the upper threshold value. If this is not the case, no event has occurred, and the method ends in step 106. If, on the other hand, the upper threshold value is exceeded, an event has occurred. It can then be assumed that air bubbles have entered the system.Accordingly, a reset of the integral part of the control should now be carried out.

[0047] However, in the event that air is present in the system, an endless loop of resets of the integral component should be avoided. To avoid this, the number of consecutive resets is limited to a maximum value n max limited, for example to n max = 3. The pressure controller includes a counter that counts the number n of consecutive resets of the integral component. A reset of the integral component is only performed as long as the number n of resets already performed is below the maximum value n max In this respect, query step 110 queries whether n < n max holds. If n < n max , a reset of the integral component of the control is performed in step 112. In step 114, the number n of resets already performed is then incremented by one.

[0048] However, if the query in query step 110 shows that the number n of resets already performed is greater than or equal to the maximum value n max , then in step 116, the counter value n, which indicates the number of resets already performed, is reset to zero, and in step 118, all further resets are blocked for a second time period t2. This second time period t2 can be set, for example, to 30 seconds.

[0049] Whenever an event is detected, a timer for the second time period t2 is started in step 120. This timer is queried in step 122. In query step 124 it is determined whether the time t since the occurrence of the event is greater than the second time period t2. If this is the case, the counter reading n, which indicates the number of resets already carried out, is reset to zero in step 126. From now on, further resets of the integral part of the control can be carried out. The purpose of the time-dependent resetting of the counter is to consider one or more air bubbles that enter the conveyor within the second time period of 30 seconds, for example, as an "event".

[0050] In Fig.4, the second time period t2 used in step 118, during which the resets of the integral component of the control are blocked, corresponds to the second time period t2 used in query step 124, after which the counter reading n is reset to zero. Alternatively to this solution, different time periods could be used for blocking the resets and for resetting the reset counter.

[0051] Fig. 5 shows a pressure curve 128 indicating the pressure in the supply line as a function of time, and a curve 130 indicating the speed of the feed pump unit 54 as a function of time. Fig. 5 shows how the exhaust aftertreatment system works when using the Fig.4, the process reacts to the appearance of air bubbles. Arrows 132 indicate the time at which the integral component of the control is reset. It can be seen that a stable pressure of the reactant in the range of ± 400 hPa around the setpoint is already achieved at the time indicated by arrow 134.

[0052] The features disclosed in the above description, the claims and the drawings may be important both individually and in any combination for the realization of the invention in its various embodiments.

Claims

[1] Method for regulating the pressure in a pressurized supply line (18) for supplying reactant for selective catalytic reduction to an exhaust gas stream (6) in an exhaust gas aftertreatment system, the exhaust gas aftertreatment system comprising: - a feed pump (54) for conveying the reactant, - the supply line (18) for supplying the reactant to the exhaust gas stream (6), wherein the supply line (18) is fluidically connected to the feed pump (54), - a pressure regulator (78) designed to adjust the delivery rate of the feed pump (54) as a function of the pressure of the reactant in the feed line (18) by means of a control having at least an integral component, the method comprising the following steps: - detecting the pressure of the reactant in the supply line (18), - detecting whether a pressure curve of the pressure of the reactant in the supply line (18) occurs which is characteristic of the occurrence of air bubbles in the reactant, - Performing a reset of the integral component depending on the detection of a pressure curve characteristic of the occurrence of air bubbles. [2] Method according to claim 1, characterized by that the control has a proportional component and an integral component. [3] Method according to one of the preceding claims, characterized by that the pressure regulator (78) is designed to regulate the pressure of the reactant in the supply line (18) to a desired pressure value. [4] Method according to one of the preceding claims, characterized by that the pressure regulator (78) is designed to regulate a speed of the feed pump (54) as a function of the pressure of the reactant in the feed line (18). [5] Method according to one of the preceding claims, characterized by that detecting the pressure curve characteristic of the occurrence of air bubbles includes: - Detecting whether a lower pressure threshold is exceeded, particularly for a specified period of time. [6] Method according to one of the preceding claims, characterized by that detecting the pressure curve characteristic of the occurrence of air bubbles includes: - Detecting whether a lower pressure threshold is first undercut for a specified period of time and later an upper pressure threshold is exceeded. [7] Method according to one of the preceding claims, characterized by that the number of consecutive resets of the integral component is limited. [8] Method according to one of the preceding claims, characterized bythat the number of resets of the integral component carried out is counted by means of a counter, whereby in the event that the number of resets carried out exceeds a predetermined maximum value, no further reset of the integral component is carried out for at least a predetermined second period of time. [9] Method according to one of the preceding claims, characterized by that if the number of resets performed exceeds a specified maximum value, a time interval is started, whereby after the time interval has elapsed the counter for the number of resets performed is reset. [10] Method according to one of the preceding claims, characterized by that the exhaust gas aftertreatment system comprises a tank (24) for the reactant. [11] Pressure regulator (78) for regulating the pressure in a pressurised supply line (18) for supplying reactant for selective catalytic reduction to an exhaust gas stream (6) in an exhaust gas aftertreatment system, wherein the exhaust gas aftertreatment system comprises a feed pump (54) for feeding the reactant, which is fluidically connected to the supply line (18), wherein the pressure regulator (78) is designed to - to regulate the delivery rate of the feed pump (54) as a function of the pressure of the reactant in the feed line (18) by means of a control system which has at least an integral component, - to detect a pressure curve of the pressure of the reactant in the supply line (18) which is characteristic of the occurrence of air bubbles in the reactant, and - to reset the integral component depending on the detection of a pressure curve characteristic of the occurrence of air bubbles. [12] Exhaust gas aftertreatment system for exhaust gas aftertreatment in an exhaust gas stream (6) by means of selective catalytic reduction, the exhaust gas aftertreatment system comprising: - a feed pump (54) for conveying the reactant, - a supply line (18) for supplying the reactant to the exhaust gas stream (6), wherein the supply line (18) is fluidically connected to the feed pump (54), - a pressure regulator (78) which is designed to regulate the delivery rate of the feed pump (54) as a function of the pressure of the reactant in the feed line (18) by means of a control which has at least an integral component, - wherein the pressure regulator (78) is designed - to detect a pressure curve of the pressure of the reactant in the supply line (18) which is characteristic of the occurrence of air bubbles in the reactant, and - to reset the integral component depending on the detection of a pressure curve characteristic of the occurrence of air bubbles. [13] An exhaust system of a motor vehicle comprising an exhaust aftertreatment system according to claim 12. [14] A motor vehicle comprising an exhaust system with an exhaust aftertreatment system according to claim 12.

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