Method and device for emptying an injection system for injecting reducing agent during engine shutdown

Abstract

The invention relates to a method, in particular a computer-implemented method, for operating a reducing agent injection system (1) with at least one metering valve (61) for an exhaust gas aftertreatment device of an engine system, comprising the following steps: - After detecting or signalling an engine stop (S1), discharging (S3) reducing agent from a pressure line (5) in successive pressure reduction phases, pre-drainage phases and draining phases; - determining a compression modulus (K) of the pressure line (5) of the reducing agent injection system (1) filled with reducing agent immediately before, during or immediately after the engine stop; - Determining the amount of reducing agent removed during the pressure reduction phase depending on the compression modulus (K), - Operating the pre-emptying phase and / or the emptying phase depending on the amount of reducing agent discharged in order to set a predetermined residual amount of reducing agent in the pressure line (5).

Description

Technical area

[0001] The invention relates to exhaust aftertreatment systems for internal combustion engines, into which reducing agents are injected using an injection system to reduce nitrogen oxides. The invention further relates to measures for draining the injection system when the engine is shut down. Technical background

[0002] When shutting down an engine system with a combustion engine and an exhaust aftertreatment system, it is necessary to bring the exhaust aftertreatment system into a state in which it is optimally protected against icing damage at low temperatures and from which exhaust aftertreatment can commence as quickly as possible upon subsequent engine start-up. Therefore, it is fundamentally important for the shutdown routine that, on the one hand, the injection valves of the injection system are largely free of reducing agent to prevent icing damage, and, on the other hand, that the pressure build-up time after a subsequent engine start-up is reduced as much as possible in order to restore the injection system to operational readiness as quickly as possible. This is achieved by ensuring that a residual amount of reducing agent remains in the supply system after the engine system is shut down. Disclosure of the invention

[0003] According to the invention, a method for emptying a reducing agent injection system of an exhaust gas aftertreatment device of an engine system after an engine stop according to claim 1 and a corresponding device according to the independent claim are provided.

[0004] Further embodiments are specified in the dependent claims.

[0005] According to a first aspect, a method, in particular an at least partially computer-implemented method, is provided for operating a reducing agent injection system for an exhaust gas aftertreatment device of an engine system, comprising the following steps: - After detecting an engine stop, draining reducing agent from the pressure line in successive pressure reduction phase, pre-drain phase and drain phase; - Determining a compression modulus of the pressure line of the reducing agent injection system filled with reducing agent immediately before (e.g. between 0 and 0.5 seconds), at or immediately after (e.g. between 0 and 0.5 seconds) the engine stop; - Determining the amount of reducing agent removed during the pressure reduction phase depending on the compression modulus, - Operating the pre-drainage phase and / or the draining phase depending on the amount of reducing agent removed in order to set a predetermined residual amount of reducing agent in the pressure line.

[0006] When the engine system is shut down, reducing agent must be removed from the injection system to prevent damage to the injection system due to icing damage in the event of freezing. Draining the injection system after an engine shutdown generally occurs in three phases: a pressure reduction phase, a pre-drain phase, and a drain phase. During all three phases, reducing agent is returned to the reducing agent tank.

[0007] However, a certain amount of reducing agent must remain in the injection system when the engine is stopped in order to build up the required pressure as quickly as possible when the engine is subsequently started. Therefore, the more reducing agent removed from the injection system when the engine is stopped, the longer it takes to build up pressure when the engine is subsequently started.

[0008] Furthermore, in the pressure reduction phase, a return valve can be opened with the metering valve closed in order to reduce the pressure in the pressure line by discharging reducing agent into a reducing agent tank, in particular to a pressure between 0 and 1 bar, in particular to between 0.3 and 0.7 bar, wherein in the pre-emptying phase, a negative pressure is generated in the pressure line by means of the reducing agent pump with the metering valve closed by pumping reducing agent back into a reducing agent tank, which is operated in the reverse direction, wherein in the emptying phase the metering valve is opened so that the reducing agent is returned from the metering valve to the reducing agent tank.

[0009] The shutdown routine initially consists of the pressure reduction phase, which is performed by opening a reducing agent pump for a return flow with the injectors closed, in order to reduce the pressure in the reducing agent line to a pressure between 0 and 1 bar, in particular to between 0.3 and 0.7 bar. The pressure reduction phase usually lasts for a period sufficient to equalize the pressure in the pressure line to approximately ambient pressure. The amount of returning reducing agent is generally not measured. The pressure reduction continues until the pressure is almost completely dissipated.

[0010] Subsequently, in the pre-emptying phase, a vacuum is generated in the injection system using the reducing agent pump, which operates in reverse. The metering valve remains closed for a predetermined time or until a certain vacuum is reached. A vacuum is generated by pumping out the reducing agent. This process is usually time-controlled, so that the pre-emptying phase ends after a predetermined time or when a certain vacuum level is reached.

[0011] Subsequently, during the emptying phase, the metering valve is opened, allowing the reducing agent to be returned from the metering valve to the reducing agent tank at a specific mass flow rate. During all steps, a specific amount of reducing agent is returned to the reducing agent tank. The amount of returned reducing agent is determined, particularly during the pressure reduction phase, by the compression modulus of the injection system and the reducing agent. The emptying phase is also controlled by time or by the number of pump cycles.

[0012] The bulk modulus is a property that indicates the stiffness of the medium in the reductant line of the injection system and the surrounding structure, which defines the volume of the reductant line. The bulk modulus is also influenced by the amount of air bubbles. Therefore, the bulk modulus, determined immediately before engine shutdown, indicates the amount of reductant returned to the reductant tank during the pressure reduction phase.

[0013] If air bubbles are present within the reducing agent in the reducing agent line, they act as an additional pressure damper and thus significantly change the compression modulus of the entire system. The more air bubbles are present in the reducing agent, the more reducing agent is discharged by the backflow during the pressure reduction phase when the reducing agent pump opens. Since the emptying of the injection system is usually time-controlled, the amount of reducing agent discharged during the pressure reduction phase and the pre-emptying phase is much higher. Therefore, the amount of reducing agent discharged from the reducing agent line cannot be precisely determined. In order to accurately determine the amount of reducing agent to be discharged, knowledge of the compression modulus of the entire reducing agent injection system is required, which takes the presence of air bubbles into account.

[0014] It can be provided that the pre-emptying phase and / or the emptying phase are carried out for a predetermined period of time, which depends on the amount of reducing agent removed during the pressure reduction phase. If the pre-emptying phase and the emptying phase are operated in a time-controlled manner, the amount of reducing agent remaining in the pressure line after the pressure reduction phase and can then be returned to the reducing agent tank depends significantly on the compression modulus. The duration of the pre-emptying phase and the emptying phase can then be adjusted depending on the amount of reducing agent removed during the pressure reduction phase.

[0015] The bulk modulus can be determined by first determining the proportion of air in the pressure line and then assigning a value of the bulk modulus depending on the proportion of air according to a given assignment function.

[0016] Furthermore, the compression modulus can be determined depending on a pressure change and a volume change of the pressure line during a pump stroke of a reducing agent pump immediately before the engine stops.

[0017] The controls are such that not all of the reducing agent is removed from the injection system. The amount of reducing agent removed is determined according to the timing controls. Furthermore, the compression modulus of the arrangement of the injection system and the reducing agent largely determines the amount of reducing agent removed. If air bubbles are present in the reducing agent in the pressure line during the emptying phase and the pre-emptying phase, the compression modulus changes considerably and more reducing agent is removed from the pressure line during the pressure reduction phase than is required to ensure adequate protection against icing damage. This unnecessarily extends the time required to restore operational readiness after an engine start, as it takes longer to build up the reducing agent pressure required to commission the exhaust aftertreatment system.

[0018] The above method therefore provides for determining the compression modulus for the injection system at the time of engine shutdown, so that the amount of reducing agent removed from the reducing agent tank during the pressure reduction phase can be more accurately determined. This makes it possible to determine the amount of reducing agent removed and to time the pre-emptying and emptying phases so that the desired amount of reducing agent is removed from the injection system.

[0019] The compression modulus K corresponds to the stiffness of the system and characterizes the system in terms of its volume and pressure, K=ΔpΔV⋅V, where Δp indicates a pressure difference in the pressure line before and after a pump horn (piston pump), the pressure difference being measured by sensors or modeled, ΔV indicates a volume change corresponding to a volume supplied by the reducing agent pump during a pump cycle, and V corresponds to a basic volume of the volume of the reducing agent line that can be filled with reducing agent.

[0020] The compression modulus K is further directly coupled to the frequency of the hydraulic system of the injection system and the speed of sound a a=K / ρ, where ρ corresponds to the density of the reducing agent.

[0021] Furthermore, there is a relationship between the speed of sound a and the resonant frequency of the injection system: f = a / (2 L), where L corresponds to the length of the reducing agent line or the length of the pressure wave path within the injection system. This illustrates the relationship between the speed of sound and the frequency of the pressure waves in the system. Monitoring the frequency can be another possible approach to maintaining the system's stiffness.

[0022] To determine the amount of reducing agent removed during the pressure reduction phase, an allocation function or an allocation table is used depending on the previously determined compression modulus. Brief description of the drawings

[0023] Embodiments are explained in more detail below with reference to the attached drawings. They show: Fig. 1 is a schematic representation of an injection system for an exhaust aftertreatment device of an engine system; and Fig. 2 a flowchart illustrating a method for operating the injection system during an engine stop. Description of embodiments

[0024] Fig. Figure 1 schematically shows a reducing agent injection system 1 for an exhaust aftertreatment device of an engine system of an internal combustion engine. The reducing agent injection system 1 serves to inject reducing agent into an exhaust tract 2 of an engine system to reduce nitrogen oxides in the combustion exhaust gas of the internal combustion engine. The reducing agent injection system 1 is operated to introduce a predetermined amount of reducing agent into the exhaust stream under the control of a control unit 10.

[0025] The reducing agent injection system 1 comprises a reducing agent tank 3 for providing reducing agent, such as aqueous urea solution (Ad Blue®), a supply module 4 with a reducing agent pump 41, a pressure sensor 42 for measuring the pressure of the reducing agent in the pressure line 5 and a pressure damping 43, a pressure line 5 and an injection unit 6 with at least one metering valve 61.

[0026] The reducing agent pump 41 serves to pump reducing agent from the reducing agent tank 3 into the pressure line 5 in order to provide the reducing agent at a predetermined pressure. The required amount of reducing agent, specified by the control unit, is metered into a mixing chamber of the exhaust tract 2 via the metering valve 61.

[0027] To prevent icing damage and to enable rapid pressure buildup after a subsequent engine start, a specific amount of reducing agent is removed from pressure line 5 when the engine is stopped. However, since the removal of the reducing agent depends significantly on the stiffness of the system, the exact amount of reducing agent cannot be determined with a time-controlled method for removing the reducing agent. Therefore, the removal amount of reducing agent is initially determined using the compression modulus.

[0028] For this purpose, a procedure is carried out when the engine stops, as shown in the flow chart of the Fig. 2 is described in more detail.

[0029] In step S1, the system first checks whether an engine stop is requested or signaled. If an engine stop or a request for an engine stop is detected, the process continues with step S2. Otherwise (alternative: No), the system returns to step S1.

[0030] In step S2 the engine system is stopped.

[0031] In step S3, the compression module K is determined depending on a pressure change Δp of the reducing agent in the pressure line measured by the pressure sensor 42 before and after a piston stroke of the reducing agent pump and a volume change ΔV of the pump stroke during a last or additionally triggered pump stroke of the reducing agent pump immediately before, during or after the engine stop, in particular if the engine stop signal was signaled: K=ΔpΔV⋅V. V corresponds to the volume of the pressure line 5, the volume of a part within the injection unit and the reducing agent pump.

[0032] In step S4, the reducing agent pump 4 is opened for a return flow to reduce the pressure in the pressure line 5 and remove a quantity of reducing agent from the pressure line 5, as pressure equalization occurs between the reducing agent system and the ambient pressure. This pressure reduction phase is carried out until the pressure has dropped to ambient pressure, thus equalizing the pressure with the pressure in the reducing agent tank 3.

[0033] The amount of reducing agent removed during this pressure reduction phase cannot be directly measured, but can be estimated using the stiffness of the injection system 1. The stiffness of the injection system 1 is specified using the compression modulus K. In step S5, the amount of reducing agent removed during the pressure reduction phase is determined based on the compression modulus K using a predefined assignment function.

[0034] In a subsequent pre-emptying phase, in step S6, reducing agent is pumped back into the reducing agent tank 3 using the reducing agent pump, with the metering valve 61 remaining closed. This creates a vacuum in the reducing agent line 5. The pre-emptying phase is carried out in a time-controlled manner. The duration of the pre-emptying phase is determined depending on the previously determined amount of reducing agent removed, in particular by applying an allocation function.

[0035] In a subsequent emptying phase, the metering valve 61 is opened in step S7 so that the reducing agent pump 61 can pump reducing agent from the pressure line 5 into the reducing agent tank 3. The duration of the active emptying phase, during which the reducing agent pump pumps reducing agent back into the reducing agent tank, is determined depending on the previously determined amount of reducing agent removed, in particular by applying a further allocation function.

Claims

[1] Method, in particular a computer-implemented method, for operating a reducing agent injection system (1) with at least one metering valve (61) for an exhaust gas aftertreatment device of an engine system, comprising the following steps: - After detecting or signalling an engine stop (S1), discharging (S3) reducing agent from a pressure line (5) in successive pressure reduction phases, pre-drainage phases and draining phases; - determining a compression modulus (K) of the pressure line (5) of the reducing agent injection system (1) filled with reducing agent immediately before, during or immediately after the engine stop; - Determining the amount of reducing agent removed during the pressure reduction phase depending on the compression modulus (K), - Operating the pre-emptying phase and / or the emptying phase depending on the amount of reducing agent discharged in order to set a predetermined residual amount of reducing agent in the pressure line (5). [2] Method according to claim 1, wherein in the pressure reduction phase a reducing agent pump (4) is opened (S4) for a return flow with the metering valve (61) closed, in order to reduce the pressure in the pressure line by discharging reducing agent into a reducing agent tank (3) to a pressure between 0 and 1 bar, in particular to between 0.3 and 0.7 bar, wherein in the pre-emptying phase a negative pressure is generated in the pressure line with the aid of the reducing agent pump (4) with the metering valve (61) closed (S6) by pumping reducing agent back into the reducing agent tank (3), which is operated in the reverse direction, wherein in the emptying phase the metering valve (61) is opened (S7) so that the reducing agent is returned from the metering valve (61) to the reducing agent tank (3). [3] Method according to claim 1 or 2, wherein the pre-emptying phase and / or the emptying phase are carried out for a predetermined period of time which depends on the amount of reducing agent discharged during the pressure reduction phase. [4] Method according to claim 3, wherein the compression modulus (K) is determined by first determining the proportion of air in the pressure line (5) and, depending on the proportion of air, a value of the compression modulus (K) is assigned according to a predetermined assignment function. [5] Method according to claim 3, wherein the compression modulus (K) is determined as a function of a pressure change and a volume change of the pressure line (5) during a pump stroke of a reducing agent pump (4) immediately before the engine stop. [6] Device adapted to carry out the method according to one of claims 1 to 5. [7] Computer program arranged to carry out the method according to one of claims 1 to 5. [8] A machine-readable storage medium on which the computer program according to claim 10 is stored.

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