Method for determining the exhaust back pressure of an engine system of a motor vehicle

Abstract

Method for determining an exhaust back pressure of an engine system (2) with an internal combustion engine (10), an exhaust system (20) which has an exhaust gas turbocharger (40) with a wastegate valve (48), and an air supply system (30) which has an adjustable throttle valve (32), comprising the following steps: detecting an exhaust gas parameter present in the exhaust gas flow direction after an exhaust gas turbocharger (40), Model-based determination of a preliminary exhaust back pressure prevailing in the exhaust gas flow direction upstream of the exhaust gas turbocharger (40) based on the exhaust gas parameter, Determination of a preload degree which depends on a boost pressure prevailing in the direction of airflow upstream of a throttle valve (32) and an intake manifold pressure prevailing in the direction of airflow downstream of the throttle valve (32), Model-based determination of a corrected exhaust back pressure by correcting the determined preliminary exhaust back pressure depending on the determined preload degree.

Description

[0001] The invention relates to a method for determining an exhaust back pressure of an engine system.

[0002] Such an engine system typically comprises an internal combustion engine with multiple cylinders, an exhaust system for discharging exhaust gases into the environment, and an air supply system for supplying air, which, in addition to the injected fuel, is required for the combustion process within the combustion chambers of the cylinders. In a supercharged internal combustion engine, the engine system additionally has a turbocharger, in particular an exhaust gas turbocharger. The exhaust gas turbocharger has a turbine wheel and a compressor wheel, which are jointly arranged on a turbocharger shaft in a rotationally fixed manner. The turbine wheel is driven by an exhaust gas mass flow emanating from the cylinders and flowing through the exhaust system, and the compressor wheel compresses the air flowing through the air supply system to the internal combustion engine. The level of compression by the turbine wheel depends on the exhaust gas mass flow flowing around the turbine wheel.To adjust the level of air compression, i.e., to adjust the boost pressure, the exhaust turbocharger features a wastegate valve. This valve includes a bypass with a switchable valve and serves to direct the exhaust gas past the turbine wheel as needed—i.e., depending on the desired boost pressure. In addition, the air supply system features an adjustable throttle valve, which allows the air mass flow to be adjusted.

[0003] During operation of an internal combustion engine, it is necessary that the ratio between the introduced air and the injected fuel corresponds as closely as possible to a predefined fuel-to-air ratio. To do this, the possible air charge of the combustion chamber must be determined, with residual gases, i.e., unexpelled exhaust gases, typically remaining in the combustion chamber. The level of residual gases remaining in the combustion chamber depends, in turn, on a so-called exhaust backpressure, which prevails in the exhaust system and at the exhaust valve of the combustion chamber. In order to determine the air charge of the combustion chamber, the exhaust backpressure must be determined as accurately as possible.

[0004] In order to improve the response of the internal combustion engine, the wastegate valve and the throttle valve are adjusted in such a way that the pressure prevailing in front of the throttle valve in the direction of air flow is considerably higher than the pressure prevailing in the direction of air flow behind the throttle valve. The pressure prevailing in front of the throttle valve is referred to below as boost pressure, which is set by the wastegate setting and thus by the exhaust gas mass flow flowing around the turbine wheel, which is set by the wastegate setting. The pressure prevailing after the throttle valve is referred to as intake manifold pressure, which is set for the various operating points of the internal combustion engine and with which the air flows into the cylinders. In order to preload the throttle valve or a preload level on the throttle valve, ieTo adjust the difference between boost pressure and intake manifold pressure, the flow cross-section can be reduced by adjusting the wastegate valve, for example, while simultaneously reducing the flow cross-section determined by the throttle valve. Adjusting the wastegate valve to preload the throttle valve changes the exhaust backpressure.

[0005] From DE 10 2006 042 872 A1 a method for determining an exhaust gas back pressure upstream of a turbine of an exhaust gas turbocharger is known, wherein firstly an exhaust gas mass flow flowing through the turbine is determined, a rotational speed of the turbine is determined, an exhaust gas pressure downstream of the turbine is determined, an exhaust gas temperature upstream of the turbine is determined and an approximate value for the exhaust gas back pressure upstream of the turbine is read out from a prepared characteristic map stored in an engine control unit and then the exhaust gas back pressure upstream of the turbine is read out from a prepared turbine characteristic map stored in the engine control unit using the determined exhaust gas mass flow, the determined rotational speed of the turbine, the determined exhaust gas pressure downstream of the turbine, the determined exhaust gas temperature upstream of the turbine and the determined approximate value for the exhaust gas back pressure upstream of the turbine.

[0006] The object of the invention is to provide a method by which the exhaust gas back pressure and thus the filling of the combustion chambers of the cylinders of the internal combustion engine can be determined simply and reliably at all operating points of the internal combustion engine.

[0007] The object of the invention is solved by the features of claim 1.

[0008] When determining exhaust backpressure, an exhaust gas parameter present downstream of the turbocharger is first recorded. Specifically, the exhaust gas parameter is measured by a sensor. The exhaust gas parameter is, in particular, an exhaust gas mass flow.

[0009] Based on the exhaust gas parameter, a preliminary exhaust gas backpressure prevailing in the direction of exhaust gas flow upstream of the turbocharger is determined using a model. The preliminary exhaust gas backpressure is preferably determined using at least one characteristic curve, in particular using a plurality of characteristic curves forming a characteristic map. Preferably, the exhaust gas mass flow, which forms the exhaust gas parameter, is detected by a sensor, and the preliminary exhaust gas backpressure in the direction of exhaust gas flow upstream of the exhaust gas turbocharger is determined by applying a characteristic curve or a characteristic map. A preferred variant is that an intermediate variable, for example an exhaust gas pressure in the direction of exhaust gas flow downstream of the exhaust gas turbocharger, is determined from the exhaust gas parameter or the exhaust gas mass flow, and the preliminary exhaust gas backpressure is determined using a characteristic curve or a characteristic map based on the determined intermediate variable.A factor is determined from the characteristic map as a function of the exhaust gas parameter and an exhaust gas pressure prevailing in the exhaust gas flow direction downstream of the exhaust gas turbocharger is multiplied by the factor in order to obtain the preliminary exhaust gas back pressure.

[0010] A preload level, i.e. the difference between the boost pressure prevailing in the air flow direction before the throttle valve and the intake manifold pressure prevailing in the air flow direction after the throttle valve, is then determined.

[0011] Finally, a corrected exhaust backpressure, which is determined by the preload of the throttle valve, is determined based on a model-based approach by correcting the already determined preliminary exhaust backpressure as a function of the determined preload level. The correction of the preliminary exhaust backpressure is preferably determined using a correction value taken from a characteristic map or characteristic curve, such that the correction value is added to the determined preliminary exhaust backpressure.

[0012] The characteristic curves or maps used can be determined by test bench tests or by simulations.

[0013] The method allows for a simple determination of an exhaust backpressure, i.e., the corrected exhaust backpressure, which corresponds to or is very close to the actual exhaust backpressure. The exhaust backpressure can be determined without taking preload into account using existing calculation tools or methods, and the change in exhaust backpressure due to preload, i.e., the corrected exhaust backpressure, can be determined simply by correcting the preliminary exhaust backpressure.

[0014] An embodiment of the invention is explained in more detail with reference to the drawing.

[0015] Figure shows schematically an engine system of a motor vehicle.

[0016] The figure shows an engine system 2 with a supercharged combustion engine 10, an exhaust system 20 and an air supply system 30.

[0017] The internal combustion engine 10 comprises a plurality of cylinders 121, 122, 123, 124, each with a combustion chamber. Each cylinder 121, 122, 123, 124 has an intake valve 141, 142, 143, 144 and an exhaust valve 161, 162, 163, 164. The intake valves 141, 142, 143, 144 and the exhaust valves 161, 162, 163, 164 are typically adjusted between an open state and a closed state via a camshaft (not shown). Furthermore, each cylinder 121, 122, 123, 124 has a fuel injector (not shown in the figure). During operation of the internal combustion engine 10, air is admitted via the intake valves 141, 142, 143, 144 and fuel is admitted via the fuel injectors into the combustion chambers of the cylinders 121, 122, 123, 124 in order to adjust the fuel-air ratio for a desired or predefined combustion.After the combustion process, the exhaust gases produced during combustion flow through the open exhaust valves 161, 162, 163, 164 from the combustion chambers of the cylinders 121, 122, 123, 124 into the exhaust system 20.

[0018] The exhaust system 20 is fluidly connected to the exhaust valves 161, 162, 163, 164 and has an exhaust gas turbocharger 40, which has a turbine wheel 44, a compressor wheel 46, and a turbocharger shaft 42. The turbine wheel 44 and the compressor wheel 46 are arranged on the turbocharger shaft 42 in a rotationally fixed manner, so that the turbine wheel 44 and the compressor wheel 46 rotate exclusively together and thus at the same speed. The exhaust gas turbocharger 40 also has a wastegate valve 48 and / or a variable turbine geometry, which is formed by a bypass 50 and a valve 52. The wastegate valve 48 serves to partially bypass the turbine wheel 44. By adjusting the wastegate valve 48, the height of the exhaust gas flowing around the turbine 44 can be adjusted and in this way a boost pressure caused by the compressor wheel 46 can be adjusted.

[0019] The air supply to the combustion chambers of cylinders 121, 122, 123, 124 is provided by the air supply system 30, which is fluidly connected to the intake valves 141, 142, 143, 144. The air supply system 30 has an adjustable throttle valve 32, by which the air mass flow to the combustion chambers of cylinders 121, 122, 123, 124 can be adjusted.

[0020] During operation of the internal combustion engine 10, the exhaust gas flowing through the exhaust system 20 flows around the turbine wheel 44 and is driven by the flowing exhaust gas. The compressor wheel 46 is arranged in an air supply duct of the air supply system 30, upstream of the throttle valve 32 in the air flow direction, and serves to compress the air supplied to the combustion chambers of the cylinders 121, 122, 123, 124.

[0021] A control unit 60 is provided to control the throttle valve 32 and the wastegate valve 48, wherein by setting the throttle valve 32 and the wastegate valve 48 in predefined positions, an air quantity dependent on the operating point of the internal combustion engine 10 is set.

[0022] In order to improve the response of the internal combustion engine 10, the wastegate valve 48 and the throttle valve 32 are adjusted at predefined operating points of the internal combustion engine 10 such that the pressure prevailing in the air flow direction upstream of the throttle valve 32 is considerably higher than the pressure prevailing in the air flow direction downstream of the throttle valve 32. The pressure prevailing in front of the throttle valve 32 is referred to below as boost pressure, which is adjusted by the wastegate setting 48 and thereby by the exhaust gas mass flow flowing around the turbine wheel 44, which is adjusted by the wastegate setting. The pressure prevailing downstream of the throttle valve 32 is referred to as intake manifold pressure, which is adjusted for the different operating points of the internal combustion engine 10 and with which the air flows into the cylinders 121, 122, 123, 124. In order to determine the degree of preload, i.e.To adjust the difference between boost pressure and intake manifold pressure, for example, the flow cross-section can be reduced by adjusting the wastegate valve 48 and, at the same time, the flow cross-section determined by the throttle valve 32 can be reduced. Adjusting the wastegate valve 48 to provide preload to the throttle valve 32 changes the exhaust back pressure.

[0023] During operation of the internal combustion engine 10, it is necessary that the ratio between the introduced air and the injected fuel corresponds as closely as possible to a predefined fuel-air ratio. For this purpose, the possible air charge of the combustion chamber of the cylinders 121, 122, 123, 124 must be determined, whereby residual gases, i.e. unexpelled exhaust gases, usually remain in the combustion chambers and influence the air charge. The level of the residual gases remaining in the combustion chamber in turn depends on a so-called exhaust gas backpressure, which prevails in the exhaust system 20 in the direction of exhaust gas flow upstream of the exhaust gas turbocharger 40 and at the exhaust valve 161, 162, 163, 164 of the combustion chamber. In order to determine the filling of the combustion chamber with air, the exhaust gas backpressure must be determined as accurately as possible.

[0024] The exhaust back pressure is determined as a function of an exhaust gas parameter, which is an exhaust gas mass flow and is detected by a sensor 62.

[0025] Based on the exhaust gas parameter, a preliminary exhaust gas backpressure prevailing in the exhaust gas flow direction upstream of the turbocharger 40 is first determined using a model, wherein the preliminary exhaust gas backpressure does not take into account the preload of the throttle valve 32. The preliminary exhaust gas backpressure is determined using a characteristic map which was determined in particular through test bench tests. The exhaust gas mass flow is detected by the sensor 62 and the exhaust gas backpressure in the exhaust gas flow direction upstream of the exhaust gas turbocharger 40 is determined directly by applying the characteristic map. Alternatively, an exhaust gas pressure in the exhaust gas flow direction downstream of the exhaust gas turbocharger can be determined from the exhaust gas parameter, i.e. from the exhaust gas mass flow, and the exhaust gas backpressure can be determined based on the exhaust gas pressure by applying a characteristic map.The characteristic map determines a factor depending on the exhaust gas parameter and an exhaust gas pressure prevailing in the exhaust gas flow direction downstream of the exhaust gas turbocharger 40 is multiplied by the factor in order to obtain the preliminary exhaust gas back pressure.

[0026] Subsequently, a preload level at the throttle valve, ie the difference between the boost pressure prevailing in the air flow direction upstream of the throttle valve 32, ie in a boost pressure range LDB, and the intake manifold pressure prevailing in the air flow direction downstream of the throttle valve 32, ie in an intake manifold pressure range SDB, is determined.

[0027] Finally, a corrected exhaust backpressure, which is determined by the preload of throttle valve 32, is determined based on a model-based approach by correcting the already determined preliminary exhaust backpressure as a function of the determined preload level. The correction of the preliminary exhaust backpressure is determined based on a characteristic map, with a corresponding correction value being taken from the characteristic map and offset against the preliminary exhaust backpressure.

[0028] This method allows for a simple determination of the exhaust backpressure, which corresponds to or is very close to the actual exhaust backpressure. The exhaust backpressure can be easily determined without taking preload into account using existing calculation tools or methods, and the change in exhaust backpressure due to preload can be determined simply by correcting the determined, preliminary exhaust backpressure.

Claims

[1] Method for determining an exhaust back pressure of an engine system (2) with an internal combustion engine (10), an exhaust system (20) which has an exhaust gas turbocharger (40) with a wastegate valve (48) and / or a variable turbine geometry, and an air supply system (30) which has an adjustable throttle valve (32), comprising the following steps: Detecting an exhaust gas parameter present in the exhaust gas flow direction downstream of an exhaust gas turbocharger (40), model-based determination of a preliminary exhaust gas back pressure prevailing in the exhaust gas flow direction upstream of the exhaust gas turbocharger (40) based on the exhaust gas parameter, Determining a degree of preload which depends on a boost pressure prevailing in the air flow direction upstream of a throttle valve (32) and an intake manifold pressure prevailing in the air flow direction downstream of the throttle valve (32), Model-based determination of a corrected exhaust back pressure by correcting the determined preliminary exhaust back pressure depending on the determined preload level. [2] Method according to claim 1, characterized by that the preliminary exhaust back pressure is determined using a characteristic curve or a characteristic map depending on the exhaust gas parameter. [3] Method according to claim 1 or 2, characterized by that the corrected exhaust back pressure is determined using a characteristic curve or a characteristic map as a function of the degree of preload. [4] Method according to one of the preceding claims, characterized by that the exhaust gas parameter is an exhaust gas mass flow. [5] Method according to one of the preceding claims, characterized by that first an exhaust gas pressure is determined in the exhaust gas flow direction after the exhaust gas turbocharger and the exhaust gas back pressure is determined based on the exhaust gas pressure.

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