Methods for determining system stiffness

Abstract

The invention relates to a method for determining the system stiffness of an injection system (1) for an aqueous urea solution for the aftertreatment of exhaust gases from an internal combustion engine, wherein the injection system (1) comprises at least one pump (4) for conveying the aqueous urea solution, at least one injector (7) for injecting the aqueous urea solution into the exhaust gas path, at least one fluid line (2) for the fluidic connection of the pump (4) to the injector (7), and at least one pressure sensor (6) configured to detect the pressure within the injection system (1), whereby the number of operating cycles of the pump (4) required to generate a predefined minimum pressure in the injection system (1), which is depressurized at the beginning of a usage cycle, is determined. The invention also relates to a device for carrying out the method.

Description

Technical field

[0001] The invention relates to a method for determining the system stiffness of an injection system for an aqueous urea solution for the aftertreatment of exhaust gases from an internal combustion engine, wherein the injection system comprises at least one pump for conveying the aqueous urea solution, at least one injector for injecting the aqueous urea solution into the exhaust gas path, at least one fluid line for the fluidic connection of the pump to the injector, and at least one pressure sensor configured to detect the pressure within the injection system. The invention also relates to a device for carrying out the method. State of the art

[0002] Worldwide, many countries have enacted legal regulations that set upper limits for the concentration of certain substances in the exhaust gases of internal combustion engines. These are mostly substances whose release into the environment is undesirable. One such substance is nitrogen oxide (NOx), the concentration of which in the exhaust gas must not exceed legally defined limits. Due to the constraints, such as the design of internal combustion engines with regard to fuel efficiency, in-engine reduction of NOx emissions is only partially effective in reducing the concentration of NOx in the exhaust gases. Therefore, exhaust aftertreatment is necessary to comply with relatively low limits.

[0003] It has been found that selective catalytic reduction (SCR) of nitrogen oxides is advantageous. This SCR method requires a nitrogen-based reducing agent. In particular, the use of ammonia (NH3) as a reducing agent has emerged as a viable alternative. Due to its chemical properties and legal regulations in many countries, ammonia is not typically stored in its pure form, as this can cause problems, especially in motor vehicles or other mobile applications. Instead of storing the reducing agent itself, reducing agent precursors are often stored and carried. A reducing agent precursor is understood to be a substance that either releases the reducing agent or can be chemically converted into it.For example, aqueous urea is a reducing agent precursor for the reducing agent ammonia.

[0004] The aqueous ammonia solution, the urea, is carried in a tank and pumped into the exhaust pipe in precisely metered quantities using a suitable pumping device. This pumping device typically includes, among other things, a pump for conveying the fluid, one or more filters for cleaning the fluid, possibly heating devices for thawing the fluid, and a control unit for processing internal and external data and for controlling the pump, the heating devices, and other controllable components, such as one or more injectors.

[0005] German patent application DE 10 2012 200 917 A1 discloses a method for detecting a blockage of one or more injection ports of a metering valve of an SCR catalyst system. This method comprises closing the metering valve, determining a first pressure profile in the hydraulic system in the delivery module of the SCR catalyst system, and determining the stiffness of the hydraulic system from the first pressure profile. Subsequently, the metering valve is opened, a second pressure profile in the hydraulic system in the delivery module of the SCR catalyst system is determined, and a blockage of one or more injection ports of the metering valve is determined from the stiffness and the second pressure profile.

[0006] DE 10 2011 105 824 B3 discloses a method for determining gas in a liquid, wherein the gas-liquid mixture is both pumped and subjected to volume and pressure changes. The pump's operating parameters, representing the volume and pressure changes, are recorded, and the gas fraction is determined from these parameters, taking into account the system compressibility. The system compressibility in the gas-filled pumping device is determined by the following steps: - Setting a starting pressure using a pressure sensor, - Recording the pump position and pressure sensor values, - Starting up a second pressure level, - Recording the pump position and pressure sensor values ​​in this second position, - Determination of the spring constant based on the pairs of values ​​and equating the spring constant determined in this way with the system compressibility.

[0007] The exact quantity of aqueous urea solution injected into the exhaust system must be known at all times to ensure optimal chemical reactions within the exhaust stream and effective reduction of nitrogen oxides. A well-established method for determining the injected quantity is to measure the pressure drop in the delivery line following injection. Combined with information about the density of the injected fluid and the system stiffness, the injected quantities can be calculated very accurately. By comparing the actual injected quantity with the desired target quantity, it can be determined whether sufficient aqueous urea has been injected.

[0008] A particular disadvantage of this approach is that the system stiffness is influenced by various hardware parameters. These include the number of injectors used, the line design, and, for example, the system's damping characteristics. Because the system stiffness depends on this multitude of factors, the determined system stiffness may be offset or generally inaccurate, thus preventing sufficiently precise control of the urea injection. Description of the invention, problem, solution, advantages

[0009] Therefore, the object of the present invention is to provide a method for determining the system stiffness of a urea injection device, which, based on less error-prone input parameters, allows for a more accurate determination of the stiffness and ideally enables the decoupling of the determination of the system stiffness from hardware factors, and in particular, hardware factors subject to tolerances. The method for determining the system stiffness should also provide a reference value for comparing system stiffnesses determined by other means.

[0010] The problem with regard to the method is solved by a method having the features of claim 1.

[0011] An embodiment of the invention relates to a method for determining the system stiffness of an injection system for an aqueous urea solution for the aftertreatment of exhaust gases from an internal combustion engine, wherein the injection system comprises at least one pump for conveying the aqueous urea solution, at least one injector for injecting the aqueous urea solution into the exhaust gas path, at least one fluid line for the fluidic connection of the pump to the injector, and at least one pressure sensor which is configured to detect the pressure within the injection system, wherein the number of pump cycles required to generate a predefined minimum pressure in the injection system, which is depressurized at the beginning of a usage cycle, is determined.

[0012] A usage cycle, meaning the system's operation during combustion engine operation, typically ends when the engine is switched off. Before the device is used again, it is in a so-called depressurized state, in which a certain amount of air is present in the fluid line and the pump is inactive. Similarly, the injector is inactive at the beginning of the usage cycle. This results in a very low pressure in the fluid line, preferably identical to the ambient pressure. Activating the pump delivers the aqueous urea solution into the fluid line, thereby increasing the pressure. The aqueous urea solution is nearly incompressible compared to the air present in the fluid line. This causes the aqueous urea solution being delivered to the fluid line to compress the air, simultaneously increasing the pressure in the fluid line.The device's pressure sensor is designed to detect the pressure prevailing in the fluid line.

[0013] The working motion of a pump, depending on its design, describes a pumping cycle for conveying the fluid. In the case of a pump with a rotatably mounted pumping element, the rotations describe the working motion; in the case of a pump with a reciprocating pumping element, the stroke movements describe the working motion. In each case, a pump-dependent delivery volume correlates with a single working motion, which is conveyed by the pump.

[0014] Starting from a generally pressureless state or at ambient pressure, the number of pump strokes required to raise the pressure in the fluid line to a certain predefined minimum level can be determined. Since the pump volume per stroke is known, the amount of fluid delivered to the fluid line can be precisely calculated by determining the number of strokes. Because the injector is inactive, no fluid exits it. The amount of air in the system is a significant factor influencing system stiffness.

[0015] It is particularly advantageous if the pump volume delivered by the pump per working movement is known in advance and stored as a characteristic value in a control unit used for controlling the injection system.

[0016] It is particularly important here that a pre-defined value must be used, depending on the system and the specific pump employed, as different pumps have different flow rates. A certain variation in the flow rate of individual pumps of the same design can be compensated for by appropriately adjusting the pump's operating strategy, speed, and control rate. The flow rate of the pumps used is particularly dependent on the prevailing temperature, pressure, and also on the pump's wear over its service life.

[0017] It is also advantageous if, from the determined work movements required to achieve the minimum pressure and the pump volume stored as a characteristic value per work movement, a delivery volume is determined which the pump had to deliver with the injector closed in order to achieve the minimum pressure in the injection system.

[0018] At the beginning of a usage cycle, a certain amount of air is contained in the fluid line. This air is highly compressible compared to the pumped fluid, so the volume occupied by the air is taken up by the pumped fluid, and the overall pressure in the injection system increases.

[0019] A preferred embodiment is characterized in that the required delivery volume is directly dependent on the amount of air present in the injection system, the amount of air being a key factor in defining the system stiffness due to its high compressibility.

[0020] It is also preferable if a statement about the stiffness of the overall system is calculated from a characteristic map of the delivery volume, whereby the characteristic map is calculated during a calibration phase via the correlation of the experimentally determined delivery volume and the system stiffness determined therefrom during the first pressure build-up in the respective usage cycle and the determined delivery volume.

[0021] Furthermore, it is advantageous if the injection system is operated alternately in two operating states, wherein the first operating state is characterized by active control of the at least one injector, and the pump is inactive in the first operating state, and the second operating state is characterized by active control of the pump, and the at least one injector is inactive.

[0022] Such an operating mode with two operating states is advantageous because the pressure in the injection system during injection, i.e., in the first operating state with the pump inactive, depends only on the metered volume. Furthermore, during the second operating state, i.e., with the pump active and the injector inactive, the pressure in the injection system depends only on the delivery volume, which is dependent on the number of operating cycles and the pump-specific pump volume. With known pressure levels in the injection system at the beginning and end of the pump activation, the hydraulic stiffness can thus be determined regularly in parallel with the requested injection.

[0023] Furthermore, it is advantageous if the pump volume, which describes the amount of fluid pumped per working movement, is adapted during the operation of the injection system.

[0024] Temperature, pressure, and wear affect the pump volume. Adjusting the pump volume used to calculate / determine the other values ​​can therefore contribute to increased accuracy over the overall service life of the injection system in general and the pump in particular.

[0025] It is also advantageous to provide a device which, for carrying out a method according to the invention, comprises at least one pump, a fluid line, an injector, and a pressure sensor, wherein the pressure sensor is designed to determine the pressure in the fluid line, and wherein the device further comprises a control device by which the pump and the injector can be controlled.

[0026] Advantageous embodiments of the present invention are described in the dependent claims and in the following description of the figures. Brief description of the drawings

[0027] The invention will now be explained in detail using exemplary embodiments and with reference to the drawings. The drawings show: Fig. 1 A schematic view of the injection system, wherein the aqueous urea solution is conveyed from a tank through a filter to a pump, along a pressure accumulator and pressure sensor to an injector, Fig. 2 three diagrams showing the two operating states according to the invention, wherein the top diagram shows the pressure in the fluid line over time, the middle diagram shows the injector control over time and the bottom diagram shows the working movements of the pump over time, Fig. 3 two diagrams, the upper diagram showing the relative system pressure over time and the lower diagram showing the number of working cycles of the pump over time, and Fig. 4 a diagram showing the system stiffness versus the delivery volume during the initial pressure build-up. Preferred embodiment of the invention

[0028] The Fig. Figure 1 shows a schematic view of the injection system 1. The injection system 1 includes a tank (not shown) for storing the aqueous urea solution. The aqueous urea solution can be transported along a fluid line 2 through a filter 3 to a pump 4. The solution is drawn from the tank by the vacuum created in the fluid line at the pump 4's inlet during operation. Downstream of the pump 4, a pressure accumulator 5 and a pressure sensor 6 are located. These sensors detect the prevailing pressure in the fluid line 2, particularly in the section of the fluid line 2 downstream of the pump 4. During operation of the pump 4, there is regularly an overpressure in this downstream section of the fluid line.

[0029] Depending on the direction of rotation of pump 4, the fluid can be pumped from the tank to the injector 7, or from the fluid line 2 upstream of the injector 7 to the tank. The preferred pumping direction is from the tank to the injector 7.

[0030] The Fig. Figure 2 shows three diagrams: 10, 11, and 12. In all three diagrams, time is plotted on the x-axis. The top diagram, 10, shows the pressure curve on the y-axis, as recorded by the pressure sensor. It can be seen that the pressure starts at 6 bar. At time t1, the pressure drops to 5.5 bar, which is reached at time t2. At time t3, the pressure rises again to 6 bar and reaches 6 bar at time t4. This pressure drop and rise repeats continuously.

[0031] Diagram 11, the middle diagram, shows the injector's operation. A value of 0 on the y-axis corresponds to a closed injector, while a value of 1 corresponds to an open injector. Diagram 1 shows two opening events, I and II, of the injector. The first opening, I, occurs at time t1 and thus triggers the pressure drop. At time t2, the injector closes again, ending the pressure drop, and shortly thereafter, at t3, a pressure increase is shown. Opening event II then also leads to a drop and later an increase in pressure.

[0032] The lower diagram 12 shows the pump activation. At a value of 0 on the y-axis, the pump is deactivated. At a value of 1 on the y-axis, the pump is activated. At time t3, the pump is activated; at this time, the pressure is at the lower level of 5.5 bar, and the injector is closed. Activating the pump increases the pressure again in diagram 10 until it finally reaches the initial level of 6 bar at time t4. The pump is deactivated again at time t4, so no further pressure increase occurs. Diagram 12 shows another activation of the pump, which also takes place with the injector closed and thus leads to another pressure increase.

[0033] The pressure levels in diagram 10 are exemplary and are intended to illustrate the basic mechanism of action and do not in any way restrict the invention with regard to the pressure ranges or functionality at other pressure levels.

[0034] The injector and pump are activated and deactivated independently. This results in two operating states: one with the injector open and the pump deactivated, and the other with the pump activated and the injector closed. This strict separation of functions ensures that the system pressure when the injector is open is determined solely by the metered amount of fluid, while the system pressure when the pump is activated is determined solely by the pump's delivery volume, which delivers fluid from the tank into the fluid line.

[0035] Fig. Figure 3 shows two further diagrams. The upper diagram, 13, shows the time course on the x-axis, while the relative system pressure is plotted on the y-axis. Graphs 14, 15, and 16 show different systems with varying system stiffnesses. Reference symbol 14 indicates a system with high system stiffness, reference symbol 15 a system with medium system stiffness, and reference symbol 16 a system with low system stiffness.

[0036] It can be seen that the stiffer the system, the faster the necessary pressure level is reached in the system.

[0037] In the lower diagram 17, the number of pump cycles is plotted on the y-axis and time on the x-axis. Graphs 18, 19, and 20 again show the respective systems with different system stiffnesses. Reference numeral 18 represents the system with high system stiffness, which generates the desired system pressure with a small number of cycles. Reference numeral 19 represents the system with medium system stiffness, which requires a higher number of cycles to achieve the system pressure. Reference numeral 20 represents a system with lower system stiffness, which requires the most cycles to reach the system pressure.

[0038] The Fig. Figure 4 shows diagram 21, which illustrates the relationship between system stiffness and delivery volume. System stiffness is plotted on the y-axis. Delivery volume is plotted on the x-axis.

[0039] The delivery volume is the volume that the respective pump can deliver in one working cycle, multiplied by the number of working cycles. The distribution in the diagram shows that the air content in the system is higher the greater the delivery volume, and the lower the system stiffness.

[0040] The examples of implementation of Fig. 1, Fig. 2, Fig. 3 to Fig. In particular, paragraph 4 does not have a restrictive character and serves to clarify the inventive idea. Reference symbol list 1 Injection system 2 Fluid line 3 filters 4 pump 5 pressure accumulators 6 Pressure sensor 7 injectors 10 Diagram 11 Diagram 12 Diagram 13 Diagram 14 Graph 15 Graph 16 Graph 17 Diagram 18 Graph 19 Graph 20 Graph 21 Diagram

Claims

[1] Method for determining the system stiffness of an injection system (1) for an aqueous urea solution for the aftertreatment of exhaust gases of an internal combustion engine, wherein the injection system (1) comprises at least one pump (4) for conveying the aqueous urea solution, at least one injector (7) for injecting the aqueous urea solution into the exhaust gas path, at least one fluid line (2) for the fluidic connection of the pump (4) with the injector (7), and at least one pressure sensor (6) which is configured to detect the pressure within the injection system (1), characterized by , that the number of pump (4) operations required to generate a predefined minimum pressure in the injection system (1), which is unpressurized at the start of a usage cycle, is determined. [2] Method according to claim 1, characterized by, that the pump volume delivered by the pump (4) per working movement is known in advance and is stored as a characteristic value in a control unit used for controlling the injection system (1). [3] Method according to any one of the preceding claims, characterized by , that from the determined work movements required to reach the minimum pressure and the pump volume stored as a characteristic value per work movement, a delivery volume is determined which the pump had to deliver with the injector (7) closed in order to reach the minimum pressure in the injection system (1). [4] Method according to claim 3, characterized by , that the required delivery volume is directly dependent on the amount of air present in the injection system (1), whereby the amount of air, due to its high compressibility, significantly defines the system stiffness. [5] Method according to any one of the preceding claims, characterized by, that a statement about the stiffness of the overall system is calculated from a characteristic map of the delivery volume, whereby the characteristic map is calculated during a calibration phase via the correlation of the experimentally determined delivery volume and the system stiffness determined therefrom during the first pressure build-up in the respective usage cycle and the determined delivery volume. [6] Method according to any one of the preceding claims, characterized by , that the injection system (1) is operated alternately in two operating states, wherein the first operating state is characterized by an active control of the at least one injector (7), and the pump (4) is inactive in the first operating state, and the second operating state is characterized by an active control of the pump (4), and the at least one injector (7) is inactive. [7] Method according to any one of the preceding claims, characterized by, that the pump volume, which describes the amount of fluid pumped per working movement, is adapted during the operation of the injection system (1). [8] Device for carrying out a method according to one of the preceding claims, comprising at least one pump (4), one fluid line (2), one injector (7), and one pressure sensor (6), wherein the pressure sensor (6) is configured to determine the pressure in the fluid line (2), wherein the device further comprises a control device by which the pump (4) and the injector (7) can be controlled.

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