Method for controlling an SCR system of an internal combustion engine having a suction line

Abstract

The invention relates to a method for controlling an SCR system of an internal combustion engine having a suction line, the SCR system comprising a pump, a suction line, and a dosing valve, a dosing agent being fed from a tank to the dosing valve by means of an actuatable pump at a predeterminable system pressure, the tank being connected to the dosing valve via the suction line, the method is characterized by the following steps: checking whether a predefinable system pressure is present for the SCR system, in particular for a predefinable time; requesting and executing at least one pump stroke, in particular a pump stroke for the pump; -wherein the pressure is continuously ascertained and stored by the control unit during the pump stroke; determining a first pressure value and a second pressure value in at least one pump stroke, wherein the first pressure value is determined at a first point in time and the second pressure value is determined at a later second point in time; -ascertaining a stiffness as a function of the first pressure value and the second pressure value; a corrected metering mass flow rate is determined by means of the stiffness and metering is carried out by means of a metering valve.

Description

Technical Field

[0001] The invention relates to a method for controlling an SCR system having an intake line of an internal combustion engine according to the preamble of claim 1 .

[0002] The invention also relates to a computer program and a computer program product which are suitable for carrying out the method. Background Art

[0003] With the so-called selective catalytic reduction technology (SCR), it is possible to reduce the nitrogen oxide emissions of internal combustion engines, especially diesel internal combustion engines, that are operated with excess air. Here, nitrogen oxides are reduced to nitrogen and water vapor, wherein either gaseous ammonia, an aqueous ammonia solution, or an aqueous urea solution is used as a reducing agent. Here, urea is used as an ammonia carrier. The reducing agent is sprayed into the exhaust gas duct of the internal combustion engine with the help of a metering system for a hydrolysis catalyst. In the hydrolysis catalyst, it is converted into ammonia by hydrolysis, and then enters the actual SCR catalyst (which is also called a DENOX catalyst) to reduce the nitrogen oxides present in the exhaust gas. The main components of this NOx reduction system are a reducing agent tank, a pump, a pressure regulator, a pressure sensor, and a metering valve. The pump delivers the reducing agent stored in the reducing agent tank to the metering valve, which is used to spray the reducing agent into the exhaust gas flow upstream of the hydrolysis catalyst. Here, the metering valve is controlled by a signal of a control device, such as a signal of a controller of the internal combustion engine, so as to deliver a certain amount of reducing agent currently required. It is preferred to use ammonia-releasing substances, such as urea, in aqueous solution, since these reducing agents can be stored and handled much more easily than, for example, gaseous ammonia. Moreover, the delivery and metering of these solutions can also be realized in a technically much simpler way than, for example, the delivery and metering of gaseous substances. Since reducing agent solutions freeze below approximately -11° C., a device for heating the reducing agent must be provided in the reducing agent container.

[0004] DE 10 2013 218 897 A1 relates to a method for quantity monitoring of a metering or injection system of an internal combustion engine, in particular of a motor vehicle, wherein the metering or injection system comprises a pipeline system containing a fluid and wherein in particular the following are provided: generating (315) a fluid pressure wave in the pipeline system; detecting (320) a pressure variation curve generated by the fluid pressure wave; determining (325) a parameter related to the propagation speed of the fluid pressure wave from the detected pressure variation curve; determining (330) the mechanical or hydraulic stiffness of the pipeline system from the determined parameter related to the propagation speed of the fluid pressure wave; and performing quantity monitoring while taking into account the stiffness of the determined pipeline system. Summary of the invention

[0005] In a first aspect, the invention relates to a method for controlling an SCR system having an intake line of an internal combustion engine.

[0006] Among them, the SCR system includes a pump, a suction pipeline, and a metering valve.

[0007] In this case, the metering agent is conveyed from a tank to a metering valve by means of a controllable pump at a predeterminable system pressure, wherein the tank is connected to the metering valve via a suction line, characterized by the following steps:

[0008] - checking whether a predefinable system pressure for the SCR system exists, in particular whether it exists for a predefinable time,

[0009] - requesting and executing at least one pump stroke, in particular a pump stroke for the pump, wherein during the pump stroke the pressure is continuously ascertained and stored by the controller,

[0010] - determining a first pressure value and a second pressure value within at least one pump stroke, wherein the first pressure value is determined at a first point in time and the second pressure value is determined at a later second point in time,

[0011] A stiffness is ascertained as a function of the first pressure value and the second pressure value, wherein a corrected metering mass flow is ascertained with the aid of the stiffness and metered via the metering valve.

[0012] The method for determining the stiffness of an SCR system having only one intake line has the particular advantage that the stiffness of the SCR system can be determined only by at least one pump stroke. The stiffness can thus be determined quickly, precisely and without waiting time.

[0013] By accurately and simply determining, a correction of the urea liquid to be dosed can be obtained, so that overdosing or underdosing of urea liquid can be minimized or prevented. Thus, a more efficient exhaust gas aftertreatment is achieved, thereby minimizing or preventing overdosing or underdosing of urea liquid. Thus, a more efficient exhaust gas aftertreatment, in particular posttreatment of NOx emissions, can be achieved. In addition, ammonia leakage from the exhaust gas channel can be prevented by overdosing of urea liquid.

[0014] In a special embodiment, the stiffness for an SCR system with an intake line is determined using the following formula:

[0015]

[0016] Where Δt=t2-t1 is the difference between the second time point and the first time point, p i is the first pressure value at the first time point (t1) and p fis a second pressure value at a second time point (t2), T is the current fluid temperature, and wherein K(T) and P(T) are temperature-related factors for the SCR system, in particular for the pump.

[0017] In another embodiment, the metering of the metering agent is ascertained by means of a metering strategy stored and calculated on the control unit.

[0018] In an advantageous embodiment, the duration between the first time and the second time corresponds to a predefinable time of at least 30 ms.

[0019] In an advantageous embodiment, the stiffness is ascertained from a plurality of pump strokes and these stiffnesses are then averaged to obtain the stiffness for the SCR system.

[0020] A more precise stiffness determination can thus be achieved.

[0021] In a special embodiment, the temperature of the fluid is determined. This temperature of the fluid is used to determine the characteristic or characteristic properties of the installed pump.

[0022] In a further aspect, the invention relates to a device, in particular a control unit, and a computer program, which are designed, in particular programmed, to carry out one of the methods. In yet another aspect, the invention relates to a machine-readable storage medium on which the computer program is stored. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Exemplary embodiments and advantageous embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description.

[0024] Figure 1 Schematically shows an SCR system with an intake line for an internal combustion engine;

[0025] Figure 2 shows a measurement example of a pressure curve for an SCR system with a suction line;

[0026] Figure 3 A first exemplary embodiment of the method according to the invention for an SCR system having an intake line is shown with the aid of a flow chart. DETAILED DESCRIPTION

[0027] Figure 1 1 shows an SCR system 25 of an internal combustion engine (not shown) with an intake line, in particular without a return line, wherein only the elements of such a reduction system that are relevant to the invention are shown here. In contrast to an SCR system with a separate return line, there is no dedicated return line, and the return of the dosing agent 105 can only be achieved via the intake line 101.

[0028] The tank 100 stores a reducing agent solution, for example an aqueous urea solution 105 , which is also marketed under the product name “AdBlue”.

[0029] The terms urea-water solution 105 , dosing agent, fluid, Adblue continue to be used as synonyms.

[0030] The urea-water solution 105 is conveyed to the dosing agent unit 300 via the line 101 by means of a pump 120 (pump mass flow ), through which the urea aqueous solution is injected into the exhaust gas channel 400 before the catalyst K (dosing mass flow ).

[0031] For this purpose, a known metering strategy is stored in the control unit 200, which is based on the exhaust gas mass flow, for example. The dosing mass flow is obtained by the temperature of the SCR catalyst (downstream of the dosing valve 310, not shown), the NOx concentration upstream and downstream of the SCR catalyst, and the ammonia level of the SCR catalyst.

[0032] The pressure in the SCR system 25 is detected by a pressure sensor 130 arranged in the line 101. The signal is converted in a pressure-voltage converter 132 and supplied to a control device 200, for example, to a motor controller. The pump 120 can also be controlled by the controller 200, more precisely, so that the pressure in the SCR system 25 remains constant. For this purpose, the system pressure p is preset for the SCR system 25. sys , the system pressure is in particular between 4 and 12 bar. The metering agent unit 300 has an actual metering valve 310 which can be actuated by an electromagnet 312 which is arranged together with the metering valve 310 in a housing 309 and by the control device 200 .

[0033] The invention is based on the recognition that SCR systems are influenced by the mechanical or hydraulic conditions of the pipeline system. These conditions can be summarized by the term stiffness. The stiffness then depends on a number of different factors: for example, the presence of air entrainment in the fluid, the strength of the pipeline, for example, as determined by the modulus of elasticity, the temperature and pressure of the fluid, and the aging and manufacturing tolerances of the components used. However, the quantitative influence of these factors on the stiffness is usually unknown or unpredictable, or can only be determined with relatively high effort.

[0034] The derivation of the method according to the invention for determining the stiffness c of an SCR system 25 with and without a return line is derived below using the continuity equation.

[0035] Here, a one-dimensional differential equation is chosen as the formula (Ansatz), in which a mean velocity field ν is assumed. This can be described as follows:

[0036]

[0037] Here, ν is the average velocity field, p is the pressure of the SCR system 25 and ρ is the density of the liquid in the SCR system. In particular, if the metering valve is connected to the suction line 101, the effective cross section of the metering valve 310 can also be used for the method. If equation (1) is used for the inflow of the fluid, in particular the urea liquid (A in ,v in ) and outflow (A out ,v out ) and the following equation is obtained:

[0038]

[0039] It is assumed that is constant throughout the system and we will Calling it the factor c, and specifically the stiffness c, we can now write the continuity equation into our final form:

[0040]

[0041] Here, c is the stiffness of the SCR system 25 and p is the pressure.

[0042] Formula (3) is used to derive the stiffness of an SCR system with only one suction line, which in particular also serves as a return line. It is assumed that the SCR system 25 maintains a stable system pressure p for a sufficiently long time. sys , a pump stroke request is requested by the controller 200. At the same time, the pressure change curve in the system is preferably continuously recorded by a pressure sensor. Assuming that the volume flow delivered by the pump to the SCR system 25 has a good approximation linear correlation (at the pressure at which the pump operates), equation 3 can be written as follows when the injector is closed:

[0043]

[0044] Wherein, c is the stiffness of the SCR system 25, and p is the system pressure.

[0045] Here, K and P are pump-specific parameters which are measured during the application phase and subsequently stored in the controller 200. These parameters are dependent on the temperature of the fluid and are therefore stored in the controller 200 as a characteristic curve.

[0046] Therefore, for the SCR system 25, the stiffness c can be transformed as follows:

[0047]

[0048] Among them, p i is the first pressure value at the first time point t1 within the pump stroke, p f is a second pressure value within the pump stroke at a second time t2, which is in particular a later time than the first time t1, and T is the current fluid temperature. Δt is the difference between the second time t2 and the first time t1.

[0049] Here, the first pressure value and the second pressure value p in the pump stroke can be i 、p f The second pressure value p is obtained. f The determination is performed at a second time t2 which is later than the first time t1. The fluid temperature T is preferably determined by the controller 200 using a temperature sensor.

[0050] K(T) and P(T) are temperature-dependent factors which describe the volume flow delivered by pump 120 per pump stroke at fluid temperature T. These temperature-dependent factors are measured during the application phase of pump 120 and are subsequently stored in controller 200 .

[0051] Figure 2 An example of a measurement of a pressure profile for an SCR system with an intake line is shown. The pressure profile p is shown over time. Starting from time t0, the constantly applied system pressure p for the SCR system 25 can be seen. sys At the starting time t start , the controller 200 sends a signal to the pump 120 of the SCR system 25 and requests at least one first pump stroke. The pump stroke ends at time t end End. At the starting time t start and the end time t end It can be seen that the pressure changes from the first pressure, especially the system pressure p sys to a higher pressure value.

[0052] During this performed pump stroke, the pressure p is continuously recorded and stored, in particular by the pressure sensor 130, by the controller 200. Furthermore, a first pressure value p is shown. i and a second pressure value p, in particular at a later point in time f Here, Δt represents the difference between the second time point t2 and the first time point t1, wherein the second pressure value p is determined at the second time point t2. f , and obtain the first pressure value p at the first time point t1 i .

[0053] Figure 3 An exemplary sequence of the method according to the invention for an SCR system 25 having an intake line 101 is shown in FIG.

[0054] In a first step 500, the activation conditions for the method are monitored in the controller 200. The method is activated when the controller 200 for the SCR system 25 detects a pressure-stable system state. If a stable system pressure p of preferably between 6 and 12 bar is successfully established, the method is activated. sys In particular, if the system pressure p reached is detected by the controller 200 within a predeterminable time, then a pressure-stable system state exists. sys , then a stable system state can exist. To this end, the controller 200 can monitor the pressure p and at a preset system pressure p sys Activation is achieved when applied within a preset time.

[0055] The method then continues in step 510 .

[0056] In step 510, at least one pump stroke is requested by the controller 200 for the pump 120 of the SCR system 25, if enabled in step 500. The pump stroke is initiated at a starting time t start Starts and ends at time t end The controller 200 continuously receives and stores the pressure via the pressure sensor 130 .

[0057] The method then continues in step 520 .

[0058] In step 520, a first pressure value p is determined at a first time point t1 within the pump stroke obtained in step 510. i And at the second time point t2, the second pressure value p is obtained. f Here, the second pressure value p at the second time point t2 is f The determination is performed at a later time than the first time t1. Preferably, there is a predeterminable time t of at least 10 ms between the first time and the second time t1, t2. app .

[0059] The method then continues in step 530 .

[0060] In step 530, the difference Δ between the second time point t2 and the first time point t1 is calculated. t In addition, the current fluid temperature T is obtained by the controller 200 .

[0061] The stiffness c of the SCR system 25 can then be calculated according to the following formula:

[0062]

[0063] The temperature-dependent factors K(T) and P(T) are ascertained in the control device 200 during the use phase of the pump 120 and are ascertained using the temperature T of the fluid.

[0064] In a special embodiment, it is also possible to obtain a plurality of values ​​for the stiffness c by repeating the pump stroke and the stiffness measurement. In a special embodiment, the stiffness values ​​determined can be averaged.

[0065] The method then continues in step 540 .

[0066] In step 540, the determined stiffness c is provided to a metering strategy for the SCR system 25 having the intake line 101. The stiffness c is used in this case to correct the metered mass flow determined by the metering strategy. Over- or under-dosing of urea liquid is thereby minimized or prevented.

[0067] Then, the corrected metering mass flow rate determined by the controller 200 is The metering is carried out via a metering valve 310 .

[0068] Therefore, more efficient exhaust gas post-treatment, especially post-treatment of NOx emissions, can be achieved. In addition, ammonia leakage from the exhaust gas passage 400 can be prevented by overdosing the urea liquid.

[0069] The method may then end or continue in step 510 .

Claims

1. A method for controlling an SCR system (25) having an intake line (101) of an internal combustion engine, in, The SCR system (25) comprises a pump (120), a suction line (101), and a metering valve (310), wherein a metering valve (310) is provided at a predeterminable system pressure (p sys ) is used to transport the dosing agent (105) from the storage tank (100) to the dosing valve (310), wherein the storage tank (100) is connected to the dosing valve (310) via the suction pipeline (101), characterized by the following steps: - checking whether a predeterminable system pressure (p ) for the SCR system (25) exists, in particular whether it exists within a predeterminable time; sys ), - requesting and executing at least one pump stroke, in particular a pump stroke for the pump (120), - wherein the pressure is continuously determined and stored by the controller 200 during the pump stroke, - determining a first pressure value (p i ) and the second pressure value (p f ), wherein the first pressure value (p i ) and at a later second time point (t2) a second pressure value (p f ), - According to the first pressure value and the second pressure value (p i 、p f ) to obtain the stiffness (c), - wherein the corrected metering mass flow is determined with the aid of the stiffness (c) And the dosing is performed through the dosing valve (310).

2. The method according to claim 1, characterized in that The stiffness (c) for an SCR system (25) having an intake line (101) is determined using the following formula: Where Δt=t2-t1 is the difference between the second time point and the first time point (t2, t1), p i is the first pressure value at the first time point (t1) and p f is a second pressure value at a second time point (t2), T is the current fluid temperature, and wherein K(T) and P(T) are temperature-related factors for the SCR system (25), in particular for the pump (120).

3. The method according to claim 1, characterized in that: The metering of the metering agent (105) is determined by means of a metering strategy stored and calculated in the controller (200).

4. The method according to claim 1, characterized in that: The duration between the first time point (t1) and the second time point (t2) corresponds to a predeterminable time (t app ).

5. The method according to claim 1, characterized in that The stiffness is taken from multiple pump strokes and then averaged to obtain stiffness (c).

6. The method according to claim 1, characterized in that Find the fluid temperature (T).

7. A computer program product comprising a computer program, characterized in that The computer program is designed to perform the steps of the method according to any one of claims 1 to 6.

8. An electronic storage medium having a computer program stored thereon, characterized in that: The computer program is designed to perform the steps of the method according to any one of claims 1 to 6.

9. A device, in particular a control unit (200), which is designed to carry out the method according to any one of claims 1 to 6.

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