Method for operating an exhaust system of an internal combustion engine

Abstract

The invention relates to a method (300) for operating an exhaust system (120) downstream of an internal combustion engine (110), wherein the exhaust system (120) comprises a secondary air feed from a secondary air system (130), wherein the method comprises a model-based determination (320) of a target value (131_1) for a secondary air pressure (131) in the secondary air system (130), wherein a physical model (200) of the exhaust system (120) used for this purpose uses a predeterminable effective flow cross-sectional area (132) of the secondary air system (130) as an application parameter and a pressure (122) within the exhaust system (120) downstream of the secondary air feed as an input variable, and the determination (320) of the target pressure (131_1) is based on a predeterminable target composition (121_1) of an exhaust gas downstream of the secondary air feed and a Composition (113) of the exhaust gas takes place upstream of the secondary air feed.Furthermore, a computing unit and a computer program for carrying out such a method are proposed.

Description

[0001] The present invention relates to a method for operating an exhaust system downstream of an internal combustion engine as well as a computing unit and a computer program for carrying out the method. Background of the invention

[0002] To achieve legally prescribed emission limits, three-way catalysts (TWC) can be used, which convert the legally regulated gaseous pollutants NO x , HC, and CO into legally non-regulated products such as N2, H2O, and CO2, and / or other catalyst systems are used. For these catalytic reactions to proceed as intended, the temperatures in the catalyst must generally exceed the so-called light-off temperature of typically 200-400°C. Once this temperature is reached or exceeded, the catalyst converts the relevant pollutants almost completely (the so-called catalyst window).

[0003] To achieve this state as quickly as possible, so-called internal catalyst heating measures can be used. For example, in a gasoline engine, efficiency is reduced by retarding the ignition angle, thereby increasing the exhaust gas temperature and the enthalpy input into the catalyst. At the same time, combustion stability can be ensured through adapted injection strategies (e.g., multiple injections).

[0004] The constant tightening of existing exhaust emissions limits and the regulation of additional pollutant components (e.g., ammonia, NH3) are leading to increasing complexity of exhaust aftertreatment systems, which typically consist of several catalysts arranged in series. Due to space constraints, catalysts in the underbody are also being used in addition to catalysts located close to the engine.

[0005] In addition to the aforementioned internal catalyst heating measures, external catalyst heating measures can also be used, for example, using electrically heated catalysts or an exhaust gas burner. Such external heating measures are described, for example, in DE 41 32 814 A1 and DE 195 04 208 A1. They are particularly suitable for quickly heating exhaust system components installed remotely from the engine to the required operating temperature, since internal engine heating measures are not effective at these locations or only take a long time to become effective.

[0006] Another measure to increase the temperature in the exhaust aftertreatment system is secondary air injection. This allows an external air mass flow to be introduced into the exhaust manifold downstream of the internal combustion engine. This air mass flow, in conjunction with incompletely combusted fuel (in the case of a rich combustion chamber lambda), causes an exothermic reaction on the hot surfaces of the manifold and turbocharger.

[0007] To determine the target value for secondary air injection, a target value for combustion chamber lambda (raw emissions of the internal combustion engine) and exhaust lambda (catalyst conversion capability) can be specified. The required secondary air quantity can be determined from the ratio of the two lambda target values and the current exhaust gas mass flow. Disclosure of the invention

[0008] According to the invention, a method for operating an exhaust system downstream of an internal combustion engine, as well as a computing unit and a computer program for implementing the method, are proposed, having the features of the independent patent claims. Advantageous embodiments are the subject of the subclaims and the following description.

[0009] The invention makes use of the measure of determining a target pressure in a secondary air system of an internal combustion engine based on a model, in particular based on a throttle model. In particular, a measured exhaust gas composition (e.g. lambda value) downstream of the internal combustion engine, a fuel mass flow supplied to the internal combustion engine and a primary air mass flow supplied to the internal combustion engine as well as at least one pressure in the exhaust system are used as input variables for the model. The pressure can be measured and / or determined based on a model. A secondary air mass flow results from a ratio between the secondary air pressure and the pressure in the exhaust system downstream of the internal combustion engine, which secondary air mass flow depends on an effective flow cross-sectional area of the secondary air path. The physical model describes in particular the relationship between the secondary air pressure and the lambda value.By inverting the physical model, the target value for the secondary air pressure can be determined based on the lambda value. This allows the target pressure to be determined based on input variables that are typically already recorded or determined in some other way, significantly reducing the required calibration effort compared to conventional solutions that determine the target pressure or another control variable of the secondary air system based on a map, for example. The effective flow cross-sectional area is the only required calibration parameter; other dependencies are automatically included.

[0010] In detail, the method for operating an exhaust system downstream of an internal combustion engine, wherein the exhaust system comprises a secondary air feed from a secondary air system, comprises a model-based determination of a target value for a secondary air pressure in the secondary air system, wherein a fluid mechanics model of the secondary air system used for this purpose uses a predeterminable effective flow cross-sectional area of the secondary air system as an application parameter and a pressure within the exhaust system downstream of the secondary air feed as an input variable, and the determination of the target pressure is based on a predeterminable target composition of an exhaust gas downstream of the secondary air feed and a composition of the exhaust gas upstream of the secondary air feed.The effective flow cross-sectional area is therefore the only required application parameter and can be easily determined, for example empirically based on the flow behavior of the secondary air system or on the basis of the component geometry.

[0011] The secondary air system itself can contain different components and actuators: On the one hand, the required secondary air can be pumped from the environment via an electric secondary air pump (SLP), and on the other hand, the required air can also be taken from the air path upstream of the internal combustion engine. The actuator can be a separate additional compressor in the secondary air duct itself or the electric / mechanical compressors already present in the air path upstream of the internal combustion engine. Furthermore, a combination of extraction from the environment and from the air path upstream of the internal combustion engine can be used. In this case, the source can be switched, although typically both sources (environment or air path) are not used simultaneously. In the following, the general term secondary air source is used as a collective term.

[0012] In at least one embodiment, the fluid mechanics model describes a relationship between the secondary air pressure and the exhaust gas composition downstream of the secondary air inlet. This is a particularly relevant relationship for the regulation or control of the exhaust system.

[0013] In at least one embodiment, the fluid mechanics model describes a relationship between a mass flow through the secondary air system and a pressure ratio of a pressure downstream of the secondary air supply to the secondary air pressure. The pressure ratio is easy to determine and has a significant influence on the actual secondary air mass flow.

[0014] In at least one embodiment, the target pressure is determined such that the predeterminable target composition downstream of the secondary air inlet is derived from the composition and an exhaust gas mass flow of the exhaust gas upstream of the secondary air inlet, as well as a secondary air mass flow resulting from the target pressure. Thus, the target value determination essentially corresponds to an inversion of the model based on the secondary air mass flow required to set the target composition.

[0015] In at least one embodiment, the method further comprises adjusting the secondary air pressure according to the determined target value. This allows the target composition to be adjusted in a targeted and reliable manner.

[0016] A computing unit according to the invention, e.g. a control unit of a motor vehicle, is configured, in particular in terms of programming, to carry out a method according to the invention.

[0017] The implementation of a method according to the invention in the form of a computer program or computer program product with program code for carrying out all method steps is also advantageous, since this entails particularly low costs, in particular if an executing control unit is also used for other tasks and is therefore already present. Finally, a machine-readable storage medium is provided with a computer program stored thereon as described above. Suitable storage media or data carriers for providing the computer program are, in particular, magnetic, optical and electrical memories, such as hard disks, flash memories, EEPROMs, DVDs, and others. Downloading a program via computer networks (Internet, intranet, etc.) is also possible. Such a download can be wired or cable-based or wireless (e.g. via a WLAN network, a 3G, 4G, 5G or 6G connection, etc.).

[0018] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0019] The invention is illustrated schematically in the drawing using an embodiment and is described below with reference to the drawing. Short description of the drawings Fig. 1 shows a schematic representation of an arrangement with an internal combustion engine and an exhaust system as can be used in an embodiment of the invention. Fig. Figure 2 shows a schematic representation of a physical model of an exhaust system as can be used in embodiments of the invention. Fig. 3 shows a schematic representation of a determination of a setpoint value for a secondary air pressure, as can be carried out within the scope of embodiments of the invention. Embodiment(s) of the invention

[0020] In Fig. 1 schematically shows an arrangement with an internal combustion engine and exhaust system, as can be used in embodiments of the invention, designated overall by 100. The internal combustion engine is designated by 110, the exhaust system by 120, and a secondary air system that can feed secondary air into the exhaust system 120 is designated by 130. Downstream of the feed point of the secondary air system 130, one or more exhaust gas aftertreatment devices, such as catalytic converters and / or particulate filters, can be provided; however, these are not shown in the figure. Typically, such components require a minimum temperature to fulfill their function.

[0021] During operation of the arrangement 100, fuel 112 and air 111 (also referred to as primary air) are supplied to the internal combustion engine and at least partially combusted in the internal combustion engine 110, producing an exhaust gas with an exhaust gas composition 113, which is expelled into the exhaust system 120 downstream of the internal combustion engine 110. Depending on the ratio between the primary air mass flow 111 and the fuel mass flow 112, the composition 113 of the exhaust gas can be adjusted within a wide range. If less primary air 111 is provided than is stoichiometrically required for the combustion of the fuel 112, the exhaust gas is referred to as a "rich" exhaust gas; if more air is provided than is required for complete combustion, the exhaust gas is referred to as a "lean" exhaust gas. A rich exhaust gas therefore still contains combustible substances that can be combusted with additional oxygen.This is used to heat the exhaust system 120 by introducing secondary air downstream of the internal combustion engine 110, by means of which the rich exhaust gas is afterburned.

[0022] The secondary air system 130 includes a secondary air source 134, by means of which secondary air is fed into the exhaust system 120. This secondary air can be taken, for example, from an air path through which the primary air 111 is fed to the internal combustion engine 110, or it can originate directly from the ambient atmosphere. A secondary air pump or a compressor, for example, can be used as a secondary air source. Particularly when taken from the primary air path, the already compressed air on the intake side from the already existing charging system can also be used.

[0023] By means of the secondary air source 134, a secondary air pressure 131 is set, in particular according to a setpoint value as determined within the scope of this invention. The determination of the setpoint value for the secondary air pressure is carried out with reference to the Fig. 2 and Fig. 3 will be explained in more detail below.

[0024] In Fig. 2 is a schematic physical model of an exhaust system, in particular the one shown in Fig. 1, as it can be used in embodiments of the invention. The physical model is designated overall by 200. As already explained, the secondary air mass flow 133 can be determined from the exhaust gas pressure 122, the secondary air pressure 131, and the secondary air temperature (here designated by 135). The secondary air mass flow 133 also depends on the temperature in the secondary air system, since the amount of substance delivered at a given pressure and volumetric output of the secondary air source is temperature-dependent. This calculation of the secondary mass flow 133 is shown in Fig. 2 with a calculation step 210, which essentially corresponds to a fluid mechanics model of the secondary air system 130:

[0025] The secondary air pressure 131 in combination with an effective cross-sectional area 132 (A eff) of the secondary air system 130 and a pressure ratio (Π) between the secondary air pressure 131 (p S ) and a pressure (p E ) in the exhaust system 120 downstream or at the secondary air inlet (∏=pEps) a secondary air mass flow 133 (ṁ S ): m˙s=Aeff⋅ps⋅2R⋅TS⋅ψ(∏S), where Ψ(Π S ) is the throttle equation of the secondary air system 130.

[0026] Typically, the effective cross-sectional area 132 of the secondary air system 130 is constant, especially if no valves are installed downstream of the secondary air source 134. In this context, it should be emphasized that the Fig. 1 merely symbolically represents the effective cross-sectional area 132 of the secondary air system 130, and a throttle valve does not actually have to be provided in the secondary air system 130. In other words, the resulting secondary air mass flow 133 depends on the secondary air pressure 131 and the exhaust gas pressure 122. The exhaust gas pressure 122 can be measured or determined based on a model.

[0027] The secondary air mass flow 133 determined in this way, in combination with the mass flows of primary air 111 supplied to the internal combustion engine (ṁ A ) and fuel 112 (ṁ F ) a total composition 121 (λ E ) of the exhaust gas downstream of the secondary air feed: λE=m˙A+m˙Sλsto⋅m˙Fwith λsto=14.7

[0028] This overall composition 121 is determined in a step 220, and in a step 230, dynamics (gas transit time 123, sensor dynamics 124, ...) of the exhaust system 120 are taken into account in order to consider the time of arrival of the exhaust gas with the determined exhaust gas composition 121 at a location within the exhaust system 120, for example, at a position of an exhaust gas sensor that determines the exhaust gas composition. In this way, the model 200 can be compared with the actual conditions and adapted if necessary, for example, to reflect aging effects in the model 200.

[0029] From an inversion of the model 200, a target value for the secondary air pressure 131 can now be determined based on the known values of the mass flows of primary air 111 and fuel 112 and a target value for the overall composition 121 of the exhaust gas. This is shown in Fig. 3 shown.

[0030] In Fig. 3 shows a schematic representation of the determination of a setpoint for the secondary air pressure 131. This determination is based, as already indicated, on an inversion of the model 200, which is Fig. 2. The inverted model, as described in Fig. 3 is designated as 300 in total. In Fig. 3, target values are marked with a suffix "_1", while actual values do not have any additional marking. Otherwise, the assignment of reference symbols to the respective physical quantities applies, as already stated in relation to Fig. 1 and Fig. 2 was used, also in relation to Fig. 3.

[0031] A target composition 113_1 of the exhaust gas downstream of the internal combustion engine 110, which may result, for example, from a heating request from a catalytic converter of the exhaust system 120, as well as a target composition 121_1 of the exhaust gas downstream of the secondary air inlet, in combination with the already known primary air mass flow 111, results in a target mass flow 133_1 with respect to the secondary air. This is determined in a step 310 of the inverted model 300. The target pressure 131_1 is then determined from the target mass flow in a second calculation step 320 of the inverted model 300, which essentially represents an inversion of the fluid mechanics model 210 of the secondary air system. For this purpose, the target mass flow is calculated from the target mass flow 133_1, taking into account the exhaust gas pressure 122, the effective cross-sectional area 132 and the temperature 135 in the secondary air system 130.

[0032] The target pressure 131_1 determined in this way can then be adjusted, for example by appropriately controlling the secondary air source 134.

[0033] It is understood that the step-by-step procedure presented here was chosen merely as an exemplary embodiment to facilitate understanding of the invention. However, embodiments of the invention may also employ a different procedure, wherein, for example, the explained steps may be performed in a different order and / or partially or completely simultaneously and / or completely or partially continuously. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 41 32 814 A1

[0005] DE 195 04 208 A1

[0005]

Claims

[1] Method (300) for operating an exhaust system (120) downstream of an internal combustion engine (110), wherein the exhaust system (120) comprises a secondary air feed from a secondary air system (130), wherein the method comprises a model-based determination (320) of a target value (131_1) for a secondary air pressure (131) in the secondary air system (130), wherein a physical model (200) of the exhaust system (120) used for this purpose uses a predeterminable effective flow cross-sectional area (132) of the secondary air system (130) as an application parameter and a pressure (122) within the exhaust system (120) downstream of the secondary air feed as an input variable, and wherein the determination (320) of the target pressure (131_1) is carried out based on a predeterminable target composition (121_1) of an exhaust gas downstream of the secondary air feed and a composition (113) of the exhaust gas upstream of the secondary air feed. [2] Method (300) according to claim 1, wherein the physical model (200) describes a relationship between the secondary air pressure (131) and the exhaust gas composition (122) downstream of the secondary air feed. [3] Method (300) according to claim 1 or 2, wherein the physical model (200) comprises a fluid mechanics model (210) which describes a relationship between a mass flow (133) through the secondary air system (130) and a pressure ratio of the pressure (122) downstream of the secondary air feed to the secondary air pressure (131). [4] Method (300) according to one of the preceding claims, wherein the target pressure (131_1) is determined such that the predeterminable target composition (121_1) downstream of the secondary air feed results from the composition (113) and an exhaust gas mass flow (111, 112) of the exhaust gas upstream of the secondary air feed, as well as a secondary air mass flow (133_1) resulting from the target pressure (131_1). [5] Method (300) according to one of the preceding claims, comprising adjusting the secondary air pressure (131) according to the determined target value (131_1). [6] Computing unit which is designed to carry out all method steps of a method according to one of the preceding claims. [7] Computer program which causes a computing unit to carry out all the method steps of a method according to one of claims 1 to 5 when it is executed on the computing unit. [8] A machine-readable storage medium having stored thereon a computer program according to claim 7.

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