Method for determining a stiffness value of a hydraulic system, a computing unit and a computer program

Abstract

The invention relates to a method for determining a stiffness value (c) of a hydraulic system (100), wherein the hydraulic system (100) has a fluid conveying device (120, 130) for conveying fluid from a fluid tank (110) to a fluid consumer (292), wherein the method comprises determining a temporal pressure curve (200) within the hydraulic system (100) upstream (123) of the fluid consumer, and determining the stiffness value (c) from the determined pressure curve (200). Furthermore, a computing unit (160) and a computer program product for carrying out the method and a corresponding hydraulic system (100) are proposed.

Description

Technical Field

[0001] The present invention relates to a method for determining the stiffness value of a hydraulic system, as well as a calculation unit and computer program for performing the method. Background Technology

[0002] Selective catalytic reduction (SCR), using ammonia (NH3) or an ammonia-releasing agent (such as urea solution), is a method for reducing nitrogen oxides (NOx) in oxygen-rich waste gases. x SCR is a promising approach. The operating window of the SCR catalytic converter, as well as its effectiveness or efficiency, is primarily determined by the physical parameters temperature and space velocity. The degree of NH3 coverage of the catalytic converter is decisive for its efficiency. To obtain the highest possible nitrogen oxide conversion, it is generally suitable to operate the SCR system at a predetermined ammonia level. To ensure an NH3 level as close as possible to the predetermined ammonia level, the amount of ammonia or ammonia-releasing reagent delivered to the system needs to be determined as precisely as possible. Summary of the Invention

[0003] According to the present invention, a method for determining the stiffness value of a hydraulic system, having the features of the independent patent claims, is provided, along with a computing unit for performing the method, a corresponding hydraulic system, and a computer program. Advantageous designs are the subject of the dependent claims and the following description.

[0004] This invention utilizes a method to determine the stiffness value of a hydraulic system by evaluating the dynamic characteristics of the pressure curve within the system. This is based on the following background: every real system consists of elastic components, and fluids, such as the urea solution mentioned earlier, also possess a certain degree of compressibility. If the fluid volume is to be measured as accurately as possible, this elasticity must be taken into account during measurement. However, for this purpose, the elasticity (or stiffness) of the system (especially the fluid within it) must be known. This knowledge is provided by means of this invention. With the precisely determined measurement quantity, particularly with the help of a consumption deviation monitor (CDM), the function of the exhaust system can be monitored, which can identify deviations between the requested fluid volume and the actual injected fluid volume (within certain predetermined limits).

[0005] In detail, according to the present invention, a method for determining the stiffness value of a hydraulic system, wherein the hydraulic system has a fluid delivery device for conveying fluid from a fluid tank to a fluid consumer, wherein the method comprises: determining a time-pressure curve within the hydraulic system upstream of the fluid consumer, and determining the stiffness value based on the determined pressure curve. It should be noted that, within the scope of the present invention, the stiffness value determined need not necessarily be directly the physical stiffness of the system. Instead, other values, such as apparent elasticity or flexibility, or the compliance of the system, can be determined as stiffness values. From such stiffness values, the physical stiffness of the system can be readily determined, and in particular calculated.

[0006] According to at least one design, the fluid comprises ammonia and / or an ammonia-releasing agent, and / or the fluid consumer has an exhaust system and / or an SCR catalytic converter downstream of the internal combustion engine, particularly in a vehicle. As already mentioned at the beginning, this is a particularly relevant application where accurate metering of at least one fluid is important.

[0007] According to at least one design, the hydraulic system is configured to at least partially and / or at least temporarily return fluid drawn from the fluid tank back to the fluid tank, rather than delivering the fluid to the fluid consumer. For example, the pump can deliver fluid from the fluid tank substantially continuously, while the valve can only transfer fluid to the fluid consumer when needed, and at least during the valve-closed phase, the fluid is returned to the fluid tank to protect the pump or to prevent it from exceeding the maximum permissible pressure in the system. Furthermore, the return can be used for temperature control of the hydraulic system, for example, to heat the fluid supplied in the fluid tank. For example, the return can be regulated by means of an overpressure valve or by an actively controllable valve. The stiffness value is determined specifically based on the pressure curve during the period when fluid is not delivered to the fluid consumer, i.e., when the total delivered fluid is returned to the fluid tank. Within the scope of the invention, this operating state is also referred to as "leerlauf." During leerlauf, the validity of the pressure curve regarding the system stiffness value is improved because the fluid curve relates only to internal system effects and not to the pressure drop caused by sending fluid to the fluid consumer.

[0008] In particular, this method may include determining a time curve of the amount of fluid returned to the fluid tank, and determining the stiffness value based on the time curve of the returned fluid amount. In other words, the amount of returned fluid can be taken into account when determining the stiffness value. Thus, a more accurate determination result is possible.

[0009] According to at least one design, the fluid delivery device has discontinuous fluid delivery characteristics, wherein the determined pressure profile includes pressure fluctuations related to these discontinuous delivery characteristics. In particular, determining the stiffness value of the system may include evaluating the amplitude and / or wavelength of the pressure fluctuations. The stiffness value can be determined particularly well from the dynamic characteristics of the pressure. Because the corresponding operating state (particularly the idling already mentioned) typically occurs frequently during the use of hydraulic systems, especially the metering devices mentioned at the beginning for supplying reductant to the (SCR) catalytic converter, this action can be used flexibly in terms of time.

[0010] According to at least one design, the method further includes manipulating the fluid delivery device to deliver a predetermined amount of fluid from the fluid tank to the fluid consumer based on a determined stiffness value. This allows for more accurate fluid metering by taking the determined stiffness value into account, which can reduce or, if necessary, completely avoid mismetering and the corresponding subsequent problems.

[0011] According to at least one design, the fluid delivery device includes a pump, particularly a piston pump, and / or a valve, particularly an injection valve. This is a particularly relevant component, especially in the aforementioned metering systems upstream of (SCR) catalytic converters, where it is frequently used for fluid delivery.

[0012] According to at least one design scheme, the method further includes determining the amount of fluid delivered to the fluid consumer based on a determined stiffness value. For example, the pressure drop during fluid delivery to the fluid consumer can be predicted based on the determined stiffness value. If the actual pressure drop deviates from the predicted pressure drop, the deviation in the amount of fluid delivered can be inferred from it. In other words, given a known stiffness value, the actual amount of fluid delivered to the fluid consumer can be inferred from the pressure drop.

[0013] In the design of this invention, the fluid volume thus determined can be used as the actual value for adjustment, which regulates the delivered fluid volume to a rated value, or generally can be used as a feedback parameter when controlling fluid injection, particularly urea injection. Furthermore, faults in the hydraulic system, such as blockages or leaks, can be identified based on the fluid volume thus determined. In the case of such identified faults, corresponding countermeasures can be taken, such as (partially) autonomously or automatically facilitating or performing maintenance of the hydraulic system.

[0014] The computing unit according to the invention, such as the control device of a motor vehicle, is particularly configured in terms of programming technology to execute the method according to the invention.

[0015] The hydraulic system according to the invention is equipped with a fluid tank, a fluid consumer, a pressure sensor upstream of the fluid consumer, and a computing unit according to the invention, wherein the system is configured to perform at least one design scheme of the method according to the invention.

[0016] It is also advantageous to implement the method according to the invention in the form of a computer program or computer program product having program code for performing all method steps, because this results in particularly low costs, especially when the control device performing the execution is also used for other tasks and therefore already exists. Finally, a machine-readable storage medium is provided having the computer program as described above stored thereon. Suitable storage media or data carriers for providing the computer program are particularly magnetic, optical, and electrical storage devices, such as hard disks, flash memory, EEPROM, DVDs, etc. Downloading the program via a computer network (Internet, intranet, etc.) is also possible. Such downloading can be performed wired, cable-connected, or wirelessly (e.g., via WLAN networks, 3G, 4G, 5G, or 6G connections, etc.).

[0017] Other advantages and design solutions of the present invention will become apparent from the specification and the accompanying drawings.

[0018] The invention is schematically illustrated in the accompanying drawings according to embodiments and is described below with reference to the accompanying drawings. Attached Figure Description

[0019] Figure 1 The schematic block diagram illustrates a design of a hydraulic system that can be used in the design scheme of this invention.

[0020] Figure 2 An exemplary pressure curve, which can be determined in the design scheme of the present invention, is shown in a schematic diagram of pressure over time.

[0021] Figure 3 The diagram illustrates the stiffness values ​​of an exemplary hydraulic system determined using the design scheme of this invention, based on the stiffness values ​​over time. Detailed Implementation

[0022] exist Figure 1 The block diagram schematically illustrates a design of a hydraulic system that can be used in the design scheme of the present invention, and is generally indicated by 100.

[0023] System 100 includes a fluid tank 110 in which a fluid, in the example shown, is provided, a liquid containing a reducing agent used in an SCR catalytic converter, such as a urea solution. The fluid tank 110 is equipped with a heated spiral tube 212 to allow for temperature regulation of the fluid, particularly for maintaining the fluid in a liquid state. A temperature sensor 117 is also provided for this purpose. In the example shown, the heated spiral tube 212 is circulated by a temperature-regulating medium (particularly liquid). Furthermore, in the example shown, the fluid tank 110 has a level sensor 115 for determining the amount of liquid contained within the fluid tank.

[0024] The fluid tank 110 is connected to a fluid delivery device via a suction line 112, which in the illustrated example has a pump 120, particularly a piston pump. The pump 120 delivers fluid from the suction line 112. Here, other components, particularly pressure and / or temperature sensors, are integrated into the pump 120, but these other components are not shown separately in the figures.

[0025] Pump 120 is connected to metering valve 130 via pressure line 123, which in the illustrated example is configured to meter fluid from pressure line 123, specifically by injection, into exhaust system 290 upstream of SCR catalytic converter 292 (which can be considered a fluid consumer). In the illustrated example, exhaust system 290 is arranged downstream of internal combustion engine 230, particularly the vehicle's drive engine, and directs at least a portion of the exhaust gas produced by internal combustion engine 230 through SCR catalytic converter 292, wherein nitrogen oxides contained in the exhaust gas are at least partially reduced. The effectiveness of this exhaust gas purification can be determined by means of exhaust gas sensor 294, which is particularly sensitive to nitrogen oxides. To control and / or monitor the function of exhaust system 290, temperature sensors 291 and 293 are also arranged upstream or downstream of SCR catalytic converter 292.

[0026] Pump 120 is also connected to fluid tank 110 via return line 121, wherein fluid is guided back from pump 120 to fluid tank 110 via return line 121, particularly when metering valve 130 is closed and / or when the pressure of fluid in pressure line 123 exceeds a threshold. For example, return line 121 may be connected directly downstream of pump 120 to pressure line 123 by means of an overpressure valve (not shown separately).

[0027] The intake line 112, return line 121, and pressure line 123 (respectively) may be equipped with heating elements 140, such as electric heating devices, resistance heating devices, or Peltier elements, to properly temperature-regulate the fluid guided therein, particularly to prevent the fluid from freezing. As already mentioned, the heating spiral tube 212 of the fluid tank 110 is heated by a temperature-regulating medium. This temperature-regulating medium can be removed from and returned to the temperature-regulating system 210, which is used to dissipate waste heat from the internal combustion engine 230. In other words, the waste heat from the internal combustion engine 230 can be at least partially used for temperature regulation of the fluid in the fluid tank 110.

[0028] System 100 includes an (electronic) computing unit 160, which, in the illustrated example, is configured to control pump 120 and metering valve 130 and is connected to these components 120, 130 in a data-transferring manner for this purpose. In the illustrated example, the computing unit 160 is powered by a battery 220. The computing unit 160 is specifically configured to perform embodiments of the method according to the invention.

[0029] The computing unit 160 can also receive and process data from sensors 115, 117, 291, 293, 294, and other sensors not explicitly shown if necessary. This data connection can be implemented partially or entirely in a wired manner (e.g., via a data bus system such as CAN) and / or wirelessly. For example, the computing unit 160 can also be configured to control the internal combustion engine 230. Alternatively, a separate computing unit can be provided for this purpose.

[0030] In particular, when the demand for reducing agent in the exhaust system 290 is determined, such as when an increased content of nitrogen oxides in the exhaust gas is measured downstream of the SCR catalytic converter 292, or when this is based on the measurement value of the exhaust gas sensor 294 which can be expected, the metering valve 130 is opened.

[0031] Nevertheless, pump 120 can still be operated substantially continuously or for a long time in order to maintain pressure within pressure line 123. Prolonged operation of the pump is often beneficial in terms of extending its lifespan as much as possible, for example, because the switching on and off processes and / or the resulting dynamic loads on the entire system 100 can lead to increased wear.

[0032] In the case of using a piston pump or other pumps with discontinuous fluid delivery characteristics, a substantially periodic pressure curve typically occurs within pressure line 123, as in Figure 2The diagram illustrates the pressure p versus time t. However, it should be noted that other pumps can also be used in principle within the scope of this invention. The pressure curve is represented here by 200. Here, pressure fluctuations occur during the operating cycle of pump 120, wherein, exemplary, a local maximum pressure is represented by p1, and a local minimum pressure immediately following it is represented by p2. The duration of the pressure transition between the maximum pressure p1 and the minimum pressure p2 is represented by Δt. In the example shown, the duration Δt is much shorter than 1 second, for example, in the range between 0.05 s and 0.1 s. For example, the operating pressure of the pressure line can be in the range between 0.8 MPa and 1.0 MPa, wherein the difference between the maximum pressure p1 and the minimum pressure p2 can be, for example, between 10 kPa and 20 kPa. In other words, the pressure fluctuations in pressure line 123 can particularly be in the single-digit percentage range of the operating pressure of pressure line 123.

[0033] It should be noted that pressure curve 200 is observed during the idling phase, i.e., during the operation phase when metering valve 130 is closed. Here, the fluid delivered by pump 120 flows back to fluid tank 110 almost entirely through return line 121. This idling phase is common in SCR catalytic converter supply systems because SCR catalytic converter 292 typically has a storage component containing ammonia and / or ammonia-releasing reagent or reducing agent, so that remetering is only required at more or less periodic intervals.

[0034] The stiffness value of system 100 can be determined from pressure curve 200, as explained in detail below.

[0035] The stiffness value (which, as explained at the beginning, can represent the system's flexibility in addition to its physical stiffness) is determined by the continuity equation, the differential form of which, assuming a one-dimensional (1-D) mean velocity field, can be expressed as follows: Here, v represents the average velocity field, p represents the pressure, and A represents the cross-sectional area of ​​the pipe in the defined part of the system under consideration.

[0036] If we now integrate over the pressure side of the entire system 100 (where not only pump 120 but also pressure line 123 are considered as the pressure side), then equation (1) can be transformed accordingly: If we now assume It is constant throughout the system, and If denoted as c, then the continuity equation in its final form can be expressed as follows: Where c represents the stiffness value of the system under consideration (especially the flexibility).

[0037] As mentioned above, system 100 is characterized by the continuous flow of fluid through return line 121. Furthermore, as in Figure 2 As shown and explained, pressure fluctuations can be observed during idling. Here, the rise in pressure curve 200 is caused by the stroke of pump 120, which delivers fluid to the pressure side of system 100. The subsequent pressure drop... This pressure drop is caused by the fluid flowing out through the return line 121. This pressure drop can be mathematically readily detected. Therefore, the evaluation of pressure curve 200 depends particularly on this segment of pressure drop between p1 and p2. This simplification improves the robustness of the method and, moreover, enables a particularly resource-efficient operating mode. Therefore, Equation 3 simplifies to: Here, A b The so-called effective cross-sectional area of ​​the return pipe 121 (in addition to the geometric cross-sectional area, the frictional component of the fluid in the return pipe 121 is also included in the effective cross-sectional area).

[0038] By integrating over Δt from p1 to p2, the analytical solution to equation 4 is obtained: For the consumption deviation monitoring (CDM) mentioned at the beginning, an effective stiffness value is required. c ana,eff ,in This leads to the following from Equation 5: .

[0039] exist Figure 3 The diagram 300 shows the stiffness value c based on time t. Figure 1 The system 100 shown uses equation (6) to determine the stiffness value based on the design scheme of the present invention. Three different systems are used here, designed for different (time-averaged) metering rates through the metering valve 130. Dots shaded in a dense, rising manner symbolize the stiffness value for a system designed for a low metering rate (e.g., 7.2 kg / h), dots shaded in a falling manner symbolize the stiffness value determined for a system designed for a high metering rate (e.g., 15 kg / h), and dots shaded in a sparse, rising manner symbolize the stiffness value determined for a system having an average metering rate between low and high metering rates (e.g., 12 kg / h). Specifically, for systems with... Figure 3 The test system illustrated in the diagram uses metering valves 130 that differ from each other. Here, in... Figure 3 As can be seen, the determination of the stiffness value is largely independent of the design metering rate used. However, as the operating duration of system 100 progresses, an increase in stiffness value (and consequently a decrease in compliance) is obtained, which can be explained by the temperature rise of system 100 and by the removal of air bubbles from the system when necessary (e.g., through air in system 100, which is expelled over time). It should be reiterated that the pressure drop after the pump stroke is considered at the operating point where metering valve 130 is closed, i.e., when metering is just not in progress.

[0040] Based on the determined stiffness value c of system 100, the actual amount of fluid being metered can be determined or corrected more accurately. Here, we consider that system 100 behaves elastically, inversely proportional to its stiffness (or substantially proportional to its flexibility), such that opening the metering valve 130 and the resulting pressure drop in the pressure line 123 leads to a decrease in the volume of the pressure line 123 and / or expansion of the fluid therein. Therefore, if the stiffness value c is high, the amount flowing through the valve may, for example, be increased because the pressure drop is partially compensated by the elastic properties. In other words, given the known stiffness value c, the expected pressure drop when the metering valve 130 is opened can be predicted. If the actual observed pressure drop and the predicted pressure drop are not sufficiently consistent, a malfunction of system 100 can be assumed. If a malfunction of system 100 is determined in this way, corresponding countermeasures can be initiated, such as performing (e.g., at least partially automatic) maintenance.

Claims

1. A method for determining a stiffness value (c) of a hydraulic system (100), wherein the hydraulic system (100) has a fluid delivery device (120, 130) for delivering fluid from a fluid tank (110) to a fluid consumer (292), wherein the method comprises determining a time-pressure curve (200) within the hydraulic system (100) upstream (123) of the fluid consumer, and determining the stiffness value (c) based on the determined pressure curve (200).

2. The method according to claim 1, wherein the hydraulic system (100) is configured to at least partially and / or at least temporarily draw (121) fluid (112) taken from the fluid tank (110) back to the fluid tank (110) instead of delivering the fluid to the fluid consumer (292), and in particular wherein the stiffness value (c) is determined based on a pressure curve (200) during the time period during which the fluid (112) taken from the fluid tank (110) is completely drawn back (121) to the fluid tank (110).

3. The method according to claim 2, wherein the method includes determining a time curve of the amount of fluid (121) returned to the fluid tank (110), and determining the stiffness value (c) based on the time curve of the amount of fluid (121) returned.

4. The method according to any one of the preceding claims, wherein the fluid delivery device (120, 130) has discontinuous fluid delivery characteristics, wherein the determined pressure curve (200) includes pressure fluctuations (p1, p2) related to the discontinuous delivery characteristics.

5. The method of claim 4, wherein determining the stiffness value (c) of the system (100) includes evaluating the amplitude and / or wavelength of the pressure fluctuations (p1, p2).

6. The method according to any one of the preceding claims, further comprising: The fluid delivery device (120, 130) is operated according to the determined stiffness value (c) to deliver a predetermined amount of fluid from the fluid tank (110) to the fluid consumer (292).

7. The method according to any one of the preceding claims, wherein the fluid delivery device (120, 130) comprises a pump (120), particularly a piston pump and / or a valve (130), particularly a jet valve.

8. The method according to any one of the preceding claims, wherein the fluid comprises ammonia and / or an ammonia-releasing agent, and / or wherein the fluid consumer has an exhaust system (290) and / or an SCR catalytic converter (292) downstream of an internal combustion engine (230) of a vehicle.

9. The method according to any one of the preceding claims further includes determining the amount of fluid supplied to the fluid consumer (292) based on the determined stiffness value (c).

10. A computing unit (160) configured to perform all method steps of the method according to any one of the preceding claims.

11. A hydraulic system (100) having a fluid tank (110), a fluid consumer (292), a pressure sensor upstream (123) of the fluid consumer (292), and a computing unit (160) according to claim 10, wherein the system (100) is configured to perform the method according to any one of claims 1 to 9.

12. A computer program, when executed on a computing unit, causes the computing unit to perform all the method steps of the method according to any one of claims 1 to 9.

13. A machine-readable storage medium having a computer program according to claim 12 stored thereon.

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