Method for removing at least a partial blockage of a metering valve of a reagent metering device due to crystal formation, device for carrying out the method, computer program and computer program product

Abstract

A method for removing reagent (14) or a precursor of a reagent that has crystallized on a metering valve (24), wherein the metering valve (24) is included in a reagent metering system (10) which meters the reagent (14) into the exhaust gas channel (16) of an internal combustion engine (18), and wherein the removal of the crystals is carried out taking into account the temperature (t) of the metering valve (24), characterized in that, following a metering operation, when the temperature (t) of the metering valve (24) falls below a temperature threshold (tSW), at least one further cleaning metering process of the metering valve (24) is initiated in order to clean the metering valve (24) of any crystals that may be present.

Description

State of the art

[0001] The invention relates to a method for removing at least a partial blockage of a metering valve of a reagent metering device due to crystal formation, which meters a reagent into an exhaust gas channel of an internal combustion engine, according to the preamble of the independent method claim.

[0002] The invention further relates to a device for carrying out the method.

[0003] The subject of this application also includes a computer program and a computer program product.

[0004] Selective catalytic reduction (SCR) can be used for exhaust gas aftertreatment. In this process, a defined quantity of a selectively acting reagent is added to the exhaust gas. The reagent can be, for example, ammonia, which is obtained from a precursor in the form of a urea-water solution in the exhaust duct through hydrolysis.

[0005] A corresponding metering device is known, for example, from DE 196 07 073 A1. The urea-water solution is conveyed through a line from a tank to a metering valve and metered into an exhaust gas channel of an internal combustion engine upstream of an SCR catalyst in order to reduce the nitrogen oxide concentration.

[0006] In current dosing systems, such as those known under the name DENOXTRONIC, a pump in a delivery module draws the urea-water solution from a reagent tank and compresses it to the system pressure intended for atomization, for example, 3 to 9 bar. Taking into account, for example, current combustion engine data and sensor data, the dosing rate is adjusted to achieve the maximum possible nitrogen oxide reduction.

[0007] German patent application DE 10 2006 044 080 A1 describes a method designed to prevent crystallization of a urea-water solution metered into the exhaust manifold of an internal combustion engine. The previously known method aims to achieve the most precise possible metering, particularly at small quantities or rates. The metering valve, implemented as a solenoid valve actuated by an electromagnet that interacts with an armature rigidly connected to a valve needle, is supplied with a pulse-width modulated metering signal that determines the metering of the urea-water solution. The opening duration of the metering valve is designed to be limited to a minimum duration at which the valve is fully open, and which is further defined to ensure the formation of a correct spray pattern during metering.This measure prevents the urea-water solution from crystallizing, which can clog the dosing valve.

[0008] German patent application DE 10 2012 200 917 A1 describes a method for detecting an existing blockage by crystallization of one or more injection ports of a metering valve of a reagent metering system by means of a urea-water solution used as a precursor of a reagent. This method includes closing the metering valve, determining an initial pressure profile in the reagent metering system, and then first determining the stiffness of the hydraulic system from this initial pressure profile. Subsequently, the metering valve is opened, a second pressure profile in the hydraulic system is determined, and a blockage of one or more injection ports of the metering valve is identified based on the stiffness and the second pressure profile.

[0009] The second pressure profile can be determined by stopping the drive motor of a feed pump in the reagent dosing system after the metering valve has opened. Alternatively, it is suggested that a sudden change in the speed of the feed pump's drive motor be introduced after the metering valve has opened.

[0010] The known method enables the detection of a blockage in a metering valve without requiring a mass flow sensor in the hydraulic system.

[0011] One described method for determining stiffness involves stopping the drive motor of the feed pump before opening the metering valve and determining the stiffness from the initial pressure curve in the hydraulic system. Alternatively, it is also possible to trigger a speed change in the drive motor of the feed pump before opening the metering valve and then determine the stiffness from the initial pressure curve in the hydraulic system.

[0012] By determining the second pressure curve in the hydraulic system after opening the metering valve, in conjunction with the stiffness of the hydraulic system, it is possible to deduce the cross-sectional area of ​​the metering valve. By comparing this determined cross-sectional area with that of an unblocked metering valve, it can be determined whether one or more of the metering valve's injection ports are blocked.

[0013] DE 10 2011 056 755 A1 discloses a method for preventing clogging of a urea injection nozzle during post-operation. In this method, the system pressure is monitored during post-operation. If a blockage is detected based on the pressure or the rate of pressure increase, the injection nozzle is repeatedly opened and closed to clear the blockage. However, triggering a cleaning process based on the metering valve temperature falling below a certain threshold after a metering operation is not described.

[0014] German patent application DE 10 2011 004 150 A1 describes a method for eliminating crystallization that has already occurred in an electromagnetically actuated metering valve that doses a urea-water solution into the exhaust manifold of an internal combustion engine. If a jamming is detected during a control unit overrun with the engine switched off by a test current applied to the metering valve, the solenoid coil of the metering valve is energized to heat the valve and thus liquefy the crystals.

[0015] The invention is based on the objective of specifying a method and a device for carrying out the method, respectively, which enable the removal of at least a partial blockage of a metering valve of a reagent metering device due to crystal formation.

[0016] The problem is solved in each case by the features specified in the subordinate claims. Disclosure of the invention

[0017] The invention relates to a method for removing reagent or a reagent precursor that has crystallized on a metering valve, wherein the metering valve is included in a reagent metering system that meters the reagent into the exhaust gas duct of an internal combustion engine, and wherein the removal of the crystals is carried out taking into account the temperature of the metering valve. The method according to the invention is characterized in that, following a metering operation, when the temperature of the metering valve falls below a temperature threshold, at least one further cleaning / metering process of the metering valve is initiated to clean the metering valve of any crystals that may be present.

[0018] The following assumes that a urea-water solution is used as the precursor for the reagent required in an SCR catalyst located in the exhaust duct to convert nitrogen oxides contained in the exhaust gas of the internal combustion engine. Instead of the urea-water solution, another reagent or a precursor to the reagent, which crystallizes under certain operating conditions, may also be used.

[0019] Under certain operating conditions, particularly those prevalent at the valve's front end, urea crystals can form in the urea-water solution dispensed by the metering valve. These crystals can prevent the reagent from being drawn back after the metering operation is switched off. This is because the crystals partially block the metering valve, preventing sufficient airflow. Reagent drawback is particularly important when there is a risk of the reagent freezing within the metering system, especially in the metering valve, to prevent damage to the system's components caused by the density change associated with freezing. A urea-water solution freezes at approximately -11 degrees Celsius.

[0020] Crystal formation can occur particularly after the reagent dosing system is switched off during the cooling process of the dosing valve, if the dosing valve is wetted with the urea-water solution, whereby the water evaporates as it passes through the temperature range, starting at temperatures of, for example, 100 degrees down to ambient temperature, leaving behind urea crystals.

[0021] The inventive method enables the removal of the crystals by means of at least one subsequent cleaning and dosing process during a control unit after-run, when the temperature of the dosing valve reaches or has already fallen below the temperature threshold.

[0022] The temperature of the metering valve is understood to be the temperature that is present in the area of ​​the front end of the metering valve, in which at least one injection opening of the metering valve is located and where crystallization of the reagent must be expected.

[0023] The inventive method avoids the need for maintenance of the reagent dosing system that would be required in the event of a malfunction. Furthermore, the inventive method ensures the formation of a correct spray cone, which on the one hand helps to prevent precipitate of the urea-water solution at the dosing valve and on the other hand ensures proper wetting of the entire front surface of the SCR catalyst.

[0024] Advantageous further developments and embodiments of the inventive method are each the subject of dependent method claims.

[0025] One design provides that a minimal amount of reagent is dosed during at least one cleaning dosing process to remove the crystals. This minimizes reagent consumption and prevents unnecessary contamination of the exhaust duct with the reagent after the regular dosing operation of the reagent dosing system has ended.

[0026] According to one embodiment, the cleaning and dosing process takes place in a temperature range of at most approximately 100 degrees Celsius or preferably below 100 degrees Celsius, since otherwise the dosing valve would immediately recrystallize due to crystallization.

[0027] In principle, the temperature at the front end of the metering valve could be measured directly or indirectly. However, one embodiment provides for the temperature of the metering valve to be calculated using a temperature model.

[0028] For example, the model can take into account a measured exhaust gas temperature upstream of the SCR catalyst to increase accuracy.

[0029] An advantageous embodiment provides that the minimum cleaning dose for removing the crystals is only administered if it has been previously determined that the metering valve is at least partially blocked by crystals. This measure also avoids the unnecessary consumption of reagent and the wetting of the exhaust duct with reagent.

[0030] A further development of the advantageous embodiment described above provides that, in order to detect at least a partial blockage of the metering valve by the crystals, the normal metering operation of the reagent metering system is switched off, that after switching off the reagent metering system, a recirculation of the reagent is provided at least from the metering valve, that the pressure of the reagent is measured at least at one point during the recirculation, that the pressure is compared with a vacuum threshold, and that if the pressure falls below the vacuum threshold, at least a partial blockage of the metering valve due to crystals is detected.

[0031] The device according to the invention for carrying out the method relates to a specially prepared control unit which contains means for carrying out the method.

[0032] The control unit preferably contains at least one electrical memory in which the process steps are stored as a control unit program.

[0033] The control unit program according to the invention provides that all steps of the method according to the invention are executed when it runs in a control unit.

[0034] The control unit program product according to the invention, comprising a program code stored on a machine-readable carrier, executes the method according to the invention when the program runs in a control unit.

[0035] The invention will be explained in more detail below with reference to an embodiment shown in the figures. Brief description of the characters Fig. Figure 1 shows a technical environment in which a method according to the invention takes place and Fig. Figure 2 shows a flowchart of the method according to the invention. Detailed description of the exemplary implementations

[0036] Fig. Figure 1 shows a reagent dosing system 10, which doses a reagent 14, or a precursor of a reagent, stored in a tank 12, into an exhaust gas duct 16 of an internal combustion engine 18 upstream of an SCR catalyst 20. The reagent 14 is pressurized by a pump 22 to a predetermined operating pressure, for example, 9 bar. The dosing rate is determined by an electrically controlled dosing valve 24.

[0037] The reagent is a liquid reagent in which crystallization can occur at the metering valve 24 under certain operating conditions. If a urea-water solution is used as a precursor to the reagent, the urea can form urea crystals.

[0038] The urea-water solution has a freezing point of -11 degrees Celsius. To prevent the reagent 14 from freezing, after the normal dosing operation of the reagent dosing system 10 is switched off during a control unit overrun, whereby the switch-off can occur simultaneously with the switch-off of the internal combustion engine 18, the reagent 14 is drawn back from the reagent dosing system 10, in particular from the metering valve 24, into the tank 12. This back-vacuum can also take place independently of the ambient temperature. The pump 22 can provide the back-vacuum pressure required for drawing back the reagent 14.

[0039] A control unit 26 is provided for controlling the reagent dosing system 10, which provides a dosing signal DV to the dosing valve 24 and a pump signal VR to the pump 22.

[0040] The dosing signal DV determines the dosing rate. The pump signal VR controls the pump 22 such that the operating pressure of, for example, 9 bar is provided during dosing operation, while when the reagent 14 is drawn back from the reagent dosing system 10, the pump 22 generates a suction pressure.

[0041] A pressure sensor 28 is provided to measure the pressure p of the reagent 14 in the reagent dosing system 10, which provides a pressure signal PS to the control unit 26.

[0042] The control unit 26 contains a metering signal setting 30, which provides the metering signal DV preferably depending on the operating conditions of the internal combustion engine 18 and / or on the condition of the exhaust gas flowing in the exhaust channel 16 and / or on the operating conditions of the SCR catalyst 20.

[0043] The control unit 26 preferably also includes a pressure regulator 32, which determines the pump signal VR depending on the detected pressure p and depending on a predetermined target pressure.

[0044] The control unit 26 preferably includes a pressure comparator 34, which compares the pressure signal PS with a vacuum threshold pSW and provides an error signal 36 depending on the comparison result. The comparison is performed when a shutdown control 38, preferably included in the control unit 26, initiates the reagent 14 to be drawn back out of the reagent dosing system 10 by means of a backflow signal RS after the normal dosing operation of the reagent dosing system 10 has been switched off.

[0045] The backflow signal RS is provided to the pressure regulator 32, which, in the event of a backflow signal RS, sets the pump signal VR in such a way that the pump 22 generates a backflow pressure that is at least sufficient to achieve a specified negative pressure.

[0046] The backflow signal RS is preferably provided to a timer 40, which is set to a predetermined delay time, for example, a few seconds. The comparison in the pressure comparator 34 takes place when a release signal FS occurs after the predetermined delay time has elapsed.

[0047] The error signal 36 is provided if the comparison shows that the pressure p of the reagent 14 falls below the vacuum threshold pSW, for example -200 mbar. In this case, it is assumed that the metering valve 24 is at least partially blocked, with the vacuum occurring due to the suction of the pump 22.

[0048] The described test to determine whether the metering valve 24 is at least partially blocked is preferably performed before the method according to the invention is carried out. In principle, the method according to the invention can be carried out independently at any time, but preferably after the regular metering operation of the reagent metering system 10 has been switched off.

[0049] According to the invention, it is provided that following a dosing operation, if the temperature t of the dosing valve 24 falls below a temperature threshold tSW, at least one cleaning dosing process of the dosing valve 24 is initiated in order to clean the dosing valve 24 of any crystals that may be present.

[0050] In principle, the shutdown control 38, which provides the backflow signal RS, can simultaneously initiate a comparison of the actual temperature t of the metering valve 24 with the temperature threshold tSW in a temperature comparator 42. Therefore, the backflow signal RS is made available to the temperature comparator 42.

[0051] Preferably, however, the temperature comparator 42 is enabled depending on the occurrence of the error signal 36. The advantage of this approach is that at least one cleaning dosing process is only carried out if at least a partial blockage of the dosing valve 24 has been detected beforehand. This avoids unnecessary dosing of the reagent 14.

[0052] The temperature comparator 42 compares the temperature threshold tSW with at least one measure of the temperature t of the metering valve 24, with particular importance being the temperature t at the point where crystallization of the reagent 14 can occur.

[0053] In principle, the temperature t of the metering valve 24 can be measured with a temperature sensor. Alternatively or additionally, it is possible to calculate the temperature of the metering valve 24 indirectly, for example by measuring the electrical resistance of a solenoid coil of the electromagnetically actuated metering valve 24.

[0054] An advantageous embodiment provides that at least one measure for the temperature t of the metering valve 24 is determined by a temperature model 44. The temperature model 44 can calculate the temperature of the metering valve 24, for example, as a function of the exhaust gas temperature, which is also calculated or measured, and the known thermal behavior of the metering valve 24.

[0055] The temperature threshold tSW is at most 100 degrees Celsius, preferably below. If the temperature t of the metering valve 24 falls below 100 degrees Celsius, corresponding to the temperature threshold tSW, the temperature comparator 42 provides a cleaning signal ES, which is made available to the shutdown control 38 and the metering signal setting 30.

[0056] The shutdown control 38 suppresses any remaining backflow signal RS and controls the pressure regulator 32 such that the pump 22 generates a pressure p of the reagent 14 suitable for cleaning dosing. Preferably, the normal operating pressure of, for example, 9 bar is set.

[0057] The dosing signal setting 30 controls the dosing valve 24 for at least one cleaning dosing process, wherein the dosed amount of reagent is preferably set to a minimum value. The minimum value is achieved with a dosing signal DV that results in the shortest possible opening time of the dosing valve 24, wherein, in the case of a pulse-width modulated dosing signal DV, a minimum pulse duration of the dosing signal DV is preferably set such that the dosing valve 24 can, in principle, open completely, so that, with a properly functioning dosing valve 24, a correct spray pattern of the sprayed reagent 14 would occur.

[0058] At least one cleaning and dosing process can already ensure that existing crystals in the dosing valve 24 or on the front area of ​​the dosing valve 24 are removed.

[0059] If necessary, the dosage of a minimum amount can be deviated from, and a longer dosing duration with a correspondingly higher dosage amount can be specified. Alternatively or additionally, it can be stipulated that several additional dosing processes be carried out.

[0060] The initiation of more than one additional dosing process can again preferably be initiated depending on a recurring error signal 36 after a further pressure test cycle has been completed.

[0061] In Fig. Figure 2 shows a flowchart of the procedure according to the invention.

[0062] The flowchart assumes that a detection of at least a partial blockage of the metering valve 24 is carried out before the method according to the invention.

[0063] In a first functional block 50, the shutdown of the normal dosing operation of the reagent dosing system 10 is defined and the shutdown control 38 is activated.

[0064] In a second functional block 52, the pump signal VR of the pump 22 is set in such a way that the reagent 14 can be drawn back out of the reagent dosing system 10.

[0065] Preferably, a delay time is specified in a third functional block 54, which is determined by the timer 40. The end of the delay time is signaled by the enable signal FS.

[0066] Subsequently, in a query 56 in the pressure comparator 34, the pressure p of the reagent 14 is compared with the vacuum threshold pSW. If the pressure p of the reagent 14 does not fall below the vacuum threshold pSW, the metering valve 24 is assumed to be functioning correctly, and the upstream verification procedure is terminated. However, if the pressure p of the reagent 14 falls below the vacuum threshold pSW, the metering valve 24 is assumed to be at least partially blocked, and the fault signal 36 is provided in a fourth functional block 58.

[0067] The error signal 36 causes the temperature comparator 42 to perform a second query 60, comparing the temperature t of the metering valve 24 with the temperature threshold tSW. The test continues if the temperature t is still too high. If the temperature t of the metering valve 24 is below the temperature threshold tSW, the temperature comparator 42 provides the cleaning signal ES in a fifth functional block 62, which triggers at least one cleaning dosing process to remove any crystals that may be present. The reagent is then removed from the reagent dosing system 10 in a second suction process. The method according to the invention is then complete.

Claims

[1] Method for removing reagent (14) or a precursor of a reagent that has crystallized on a metering valve (24), wherein the metering valve (24) is included in a reagent metering system (10) which meters the reagent (14) into the exhaust duct (16) of an internal combustion engine (18) and wherein the removal of the crystals is carried out taking into account the temperature (t) of the metering valve (24), characterized by , that following a dosing operation, if the temperature (t) of the dosing valve (24) falls below a temperature threshold (tSW), at least one cleaning dosing process of the dosing valve (24) is initiated to clean the dosing valve (24) of any crystals that may be present. [2] Method according to claim 1, characterized by this, characterized by this , that in at least one cleaning dosing process a minimal amount of the reagent (14) is dosed. [3] Method according to claim 1, characterized bythat the temperature threshold (tSW) is a maximum of 100 degrees Celsius. [4] Method according to claim 1, characterized by , that the temperature (t) of the metering valve (24) is calculated using a temperature model (44). [5] Method according to claim 1, characterized by , that an investigation of at least partial blockage of the metering valve (24) due to crystallized reagent (14) is planned. [6] Method according to claim 5, characterized by, that to determine whether the metering valve (24) is at least partially blocked by crystals, the reagent metering system (10) is switched off, that after switching off the reagent metering system (10) the reagent (14) is drawn back out of the metering valve (24), that the pressure (p) of the reagent (14) is measured at least at one point during the drawing back, that the pressure (p) is compared with a vacuum threshold (pSW), and that if the pressure (p) falls below the vacuum threshold (pSW), at least a partial blockage of the metering valve (24) by crystals is detected. [7] Device for operating a reagent dosing device, characterized by , that at least one control unit (26) specifically designed for carrying out the method according to one of claims 1-6 is provided. [8] Computer program that performs all the steps of a method according to any one of claims 1-6 when run on a computer. [9] Computer program product comprising program code stored on a machine-readable medium for carrying out the method according to any one of claims 1-6 when the program is executed on a computer.

Images