Method for determining a system rigidity

Abstract

The invention relates to a method for determining a system rigidity 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) has 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 line, at least one fluid line (2) for fluidically connecting the pump (4) to the injector (7), and at least one pressure sensor (6) which is configured to detect the pressure within the injection system (1), and the number of working movements that need to be carried out by the pump (4) in order to generate a defined minimum pressure in the injection system (1), which is unpressurized at the start of a use cycle, is determined. The invention also relates to a device for carrying out the method.

Description

[0001] Description

[0002] Method for determining system stiffness

[0003] Technical area

[0004] 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. 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 fluidically connecting the pump to the injector, and at least one pressure sensor configured to detect the pressure within the injection system. Furthermore, the invention relates to a device for implementing the method.

[0005] State of the art

[0006] Many countries around the world have enacted legal regulations that set upper limits for the content of certain substances in the exhaust gases of internal combustion engines. These are usually substances whose release into the environment is undesirable. One of these substances is nitrogen oxide (NOx), the proportion of which in the exhaust gas must not exceed legally stipulated limits. Due to the framework conditions, for example the design of internal combustion engines with a view to low fuel consumption or similar, internal engine-based prevention of nitrogen oxide emissions is only of limited use in reducing the proportion of nitrogen oxides in the exhaust gas. Therefore, exhaust gas aftertreatment is required to comply with relatively low limits.

[0007] It has been shown that selective catalytic reduction (SCR) of nitrogen oxides is advantageous. This SCR method requires a reducing agent that contains nitrogen. In particular, the use of ammonia (NH3) as a reducing agent has emerged as a possible alternative. Due to its chemical properties and legal regulations in many countries, ammonia is not usually stored in pure form, as this can lead to problems, particularly in motor vehicles or other mobile applications. Instead of storing the reducing agents themselves, reducing agent precursors are often stored and carried along. A reducing agent precursor is understood to be a substance that splits off the reducing agent or can be chemically converted into the reducing agent.For example, for the reducing agent ammonia, aqueous urea is a reducing agent precursor.

[0008] The aqueous ammonia solution, the urea, is carried in a tank and pumped into the exhaust line in precisely measured quantities using a suitable pumping device. The pumping device typically includes, among other things, a pump for pumping the fluid, one or more filters for cleaning the fluid, optionally heating devices for thawing the fluid, and a control device for processing internal and external data and controlling the pump, the heating devices, and other controllable components, such as one or more injectors.

[0009] The exact amount of aqueous urea solution delivered into the exhaust line must be known at all times to ensure optimal chemical reactions in the exhaust system and effective reduction of nitrogen oxides. A well-known method for determining the injected amount is measuring the pressure drop in the delivery line due to the injection. Combined with information about the density of the delivered fluid and the system stiffness, the injected amounts can be calculated very precisely. By comparing the actual injected amount with the desired target amount, it can be determined whether sufficient aqueous urea has been injected. A particular disadvantage of this is that the system stiffness is influenced by different hardware parameters.These include the number of injectors used, the line design, and, for example, the damping characteristics of the system. Since system stiffness depends on these numerous factors, the system stiffness determined in this way may be offset or generally inaccurate, making sufficiently precise control of urea injection impossible.

[0010] Description of the invention, task, solution, advantages

[0011] Therefore, the object of the present invention is to provide a method for determining the system stiffness of a urea injection device that allows a more precise determination of the stiffness based on less error-prone input variables and ideally enables a decoupling of the determination of the system stiffness from hardware factors, particularly those subject to tolerances. The method for determining the system stiffness should also provide a reference value for comparing system stiffnesses determined by other methods.

[0012] The problem with regard to the method is solved by a method having the features of claim 1.

[0013] One embodiment of the invention relates to a method for determining a system stiffness of an injection system for an aqueous urea solution for the aftertreatment of exhaust gases of an internal combustion engine, wherein the injection system has 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 fluidically connecting the pump to the injector and at least one pressure sensor which is designed to detect the pressure within the injection system, wherein the number of working movements of the pump is determined which is required in order to generate a predefined minimum pressure in the injection system, which is depressurized at the start of a usage cycle.

[0014] A usage cycle, i.e. the use of the system when the combustion engine is running, usually ends when the combustion 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 not active. Likewise, the injector is not active at the start of the usage cycle. As a result, the pressure in the fluid line is very low, which is preferably identical to the ambient pressure. By activating the pump, the aqueous urea solution is pumped into the fluid line, thus increasing the pressure in the fluid line. The aqueous urea solution is almost incompressible compared to the air in the fluid line. This means that the aqueous urea solution pumped into the fluid line compresses the air and, at the same time, the pressure in the fluid line increases.The pressure sensor of the device is designed to detect the pressure prevailing in the fluid line.

[0015] Depending on its design, the working motion of a pump describes the pumping action required to convey the fluid. In the case of a pump with a rotatably mounted discharge section, the revolutions describe the working motion; in the case of a pump with a discharge section that can be moved up and down, the reciprocating movements describe the working motion. In each case, a pump-dependent displacement volume is correlated with each individual working motion.

[0016] Starting from a generally pressureless state or from a pressure at ambient level, the number of operating movements required to raise the pressure in the fluid line to a certain predefined minimum level can be determined. Since the pump volume per operating movement is known in advance, the amount of fluid pumped into the fluid line can be precisely determined by determining the operating movements. Since the injector is inactive, no fluid escapes from the injector. The amount of air in the system is a key factor influencing system stiffness.

[0017] It is particularly advantageous if the pump volume delivered by the pump per working movement is known in advance and is stored as a characteristic value in a control unit used to control the injection system.

[0018] It is particularly important here that a known value must be used, depending on the system and the pump used, as different pumps have different flow rates. A certain variation in the flow rate of individual pumps of the same type can be compensated for by appropriately adjusting the pump's operating strategy, speed, and control speed. The pumps used exhibit a particular dependence of the respective flow rate on the prevailing temperature, the prevailing pressure, and also the wear and tear of the pump over its lifetime.

[0019] It is also advantageous if, from the determined working movements required to reach the minimum pressure and the pump volume stored as a characteristic value per working movement, a delivery volume is determined which the pump had to deliver with the injector closed in order to reach the minimum pressure in the injection system.

[0020] At the beginning of a usage cycle, the fluid line contains a certain amount of air. This air is highly compressible compared to the pumped fluid, so the volume occupied by the air is taken up by the pumped fluid, increasing the overall pressure in the injection system.

[0021] A preferred embodiment is characterized in that the required delivery volume is directly dependent on the air volume present in the injection system, whereby the air volume, due to its high compressibility, significantly defines the system stiffness. 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 between the experimentally determined delivery volume and the system stiffness determined from it during the first pressure build-up in the respective usage cycle and the determined delivery volume.

[0022] Furthermore, it is advantageous if the injection system is operated alternately in two operating states, wherein the first operating state is characterized by an 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 an active control of the pump and the at least one injector is inactive.

[0023] 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, the pressure in the injection system during the second operating state—i.e., with the pump active and the injector inactive—depends only on the delivery volume, which is dependent on the number of working movements and the pump-specific pump volume. With known pressure levels in the injection system at the beginning and end of pump control, the hydraulic stiffness can thus be determined regularly in parallel with the requested injection.

[0024] Furthermore, it is advantageous if the pump volume, which describes the amount of fluid delivered per working movement, is adapted during operation of the injection system.

[0025] Temperature, pressure, and wear influence the pump volume. Adjusting the pump volume used to calculate / determine the other values ​​can thus contribute to increased accuracy over the entire service life of the injection system in general and of the pump in particular. It is also expedient to provide a device that, for carrying out a method according to the invention, has at least one pump, a fluid line, an injector, and a pressure sensor. The pressure sensor is designed to determine the pressure in the fluid line. The device further comprises a control device via which the pump and the injector can be controlled.

[0026] Advantageous further developments of the present invention are described in the subclaims and in the following description of the figures.

[0027] Short description of the drawings

[0028] The invention is explained in detail below using exemplary embodiments with reference to the drawings. In the drawings:

[0029] Fig. 1 is 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,

[0030] Fig. 2 shows three diagrams showing the two operating states according to the invention, the uppermost diagram showing the pressure in the fluid line over time, the middle diagram showing the injector control over time and the lower diagram showing the working movements of the pump over time,

[0031] Fig. 3 shows two diagrams, the upper diagram showing the relative system pressure over time and the lower diagram showing the number of working movements of the pump over time, and Fig. 4 is a diagram showing the system stiffness over the discharge volume during the initial pressure build-up.

[0032] Preferred embodiment of the invention

[0033] Figure 1 shows a schematic view of the injection system 1. The injection system 1 has 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 suction from the tank occurs via the negative pressure that is generated in the fluid line on the inlet side of the pump 4 when the pump 4 is operating. Downstream of the pump 4 in the direction of flow are a pressure accumulator 5 and a pressure sensor 6, which detect the prevailing pressure in the fluid line 2, in particular in the section of the fluid line 2 that is downstream of the pump 4 in the direction of flow. In the section of the fluid line downstream of the pump 4 in the direction of flow, an overpressure regularly prevails when the pump 4 is operating.

[0034] Depending on the direction of rotation of pump 4, the fluid can be pumped from the tank to injector 7, or from the fluid line 2 upstream of injector 7 to the tank. The preferred pumping direction is from the tank to injector 7.

[0035] Figure 2 shows three diagrams 10, 11 and 12. In all three diagrams 10, 11 and 12 the time is plotted on the x-axis. The upper diagram 10 shows the pressure curve which is recorded by the pressure sensor on the y-axis. It can be seen that the pressure has its starting level at the level 6 bar. At time t1 the pressure drops to the level 5.5 bar, which is reached at time t2. At time t3 the pressure rises again to the level of 6 bar and reaches the pressure of 6 bar at time t4. The lowering and rising of the pressure occurs repeatedly in a continuous manner. The middle diagram 11 shows the actuation of the injector. The value 0 on the y-axis corresponds to a closed injector, while the value 1 on the y-axis corresponds to an open injector. Diagram 1 shows two opening processes 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, stopping the pressure drop. Shortly thereafter, at t3, a pressure increase is observed again. Opening process II then also leads to a further decrease and later increase in pressure.

[0036] The lower diagram 12 shows the activation of the pump. 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 t3, the pressure is at the lower level of 5.5 bar and the injector is closed. By activating the pump, the pressure in diagram 10 increases again until it finally reaches the initial level of 6 bar at time t4. The pump is deactivated again at time t4, so that no further pressure increase occurs. Diagram 12 shows another activation of the pump, which also occurs with the injector closed and thus again leads to an increase in pressure.

[0037] The pressure levels in diagram 10 are exemplary and are intended to illustrate the basic mechanism of action and in no way limit the invention with regard to the pressure ranges or functionality at other pressure levels.

[0038] The injector and pump are activated and deactivated independently of one another. This results in two operating states: one showing an open injector with the pump deactivated, and one showing an activated pump with 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 is pumped from the tank into the fluid line. 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 differing system stiffnesses.The reference number 14 shows a system with high system stiffness, the reference number 15 shows a system with medium system stiffness and the reference number 16 shows a system with low system stiffness.

[0039] It can be seen that the required pressure level in the system is reached more quickly, the stiffer the system is.

[0040] In the lower diagram 17, the number of working movements of the pump is plotted on the Y-axis and the time is also plotted on the X-axis. Graphs 18, 19 and 20 again show the respective systems with different system stiffnesses. Reference number 18 represents the system with high system stiffness, which generates the desired system pressure with a small number of working movements. Reference number 19 shows the system with medium system stiffness, which requires a higher number of working movements to reach the system pressure. Reference number 20 shows a system with lower system stiffness, which requires the most working movements to reach the system pressure.

[0041] Figure 4 shows a diagram 21 depicting the relationship between system stiffness and displacement. The system stiffness is plotted on the Y-axis. The displacement is plotted on the X-axis.

[0042] The displacement is the volume that the respective pump can deliver with one working movement, multiplied by the number of working movements. The distribution in the diagram shows that the larger the displacement, the higher the air content in the system, and the lower the system stiffness. The exemplary embodiments in Figures 1 to 4 are not limiting in nature and serve to illustrate the inventive concept.

[0043] List of reference symbols

[0044] 1 . Injection system

[0045] 2. Fluid line

[0046] 3. Filter

[0047] 4. Pump

[0048] 5. Pressure accumulator

[0049] 6. Pressure sensor

[0050] 7. Injector

[0051] 10. Diagram

[0052] 11. Diagram

[0053] 12. Diagram

[0054] 13. Diagram

[0055] 14. Graph

[0056] 15. Graph

[0057] 16. Graph

[0058] 17. Diagram

[0059] 18. Graph

[0060] 19. Graph

[0061] 20. Graph

[0062] 21. Diagram

Claims

Patent claims 1. Method for determining a 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) has 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 fluidically connecting the pump (4) to the injector (7) and at least one pressure sensor (6) which is designed to detect the pressure within the injection system (1), characterized in that the number of working movements of the pump (4) is determined which is required in order to generate a predefined minimum pressure in the injection system (1), which is depressurized at the start of a usage cycle.

2. Method according to claim 1, characterized in 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 one of the preceding claims, characterized in that from the determined working movements which are required to reach the minimum pressure and the pump volume stored as a characteristic value per working 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 in that the necessary delivery volume is directly dependent on the amount of air present in the injection system (1), the amount of air decisively defining the system rigidity due to its high compressibility.

5. Method according to one of the preceding claims, characterized in that a statement about the stiffness of the overall system is calculated from a characteristic map of the delivery volume, wherein the characteristic map is calculated during a calibration phase via the correlation of the delivery volume determined experimentally 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 one of the preceding claims, characterized in 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 one of the preceding claims, characterized in that the pump volume, which describes the quantity of fluid delivered per working movement, is adapted during operation of the injection system (1).

8. Device for carrying out a method according to one of the preceding claims, with at least one pump (4), a fluid line (2), an injector (7), and a pressure sensor (6), wherein the pressure sensor (6) is designed to determine the pressure in the fluid line (2), wherein the device further comprises a control device via which the pump (4) and the injector (7) can be controlled.

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