Method and apparatus for operating a turbocharged internal combustion engine

Abstract

Method for operating an internal combustion engine (2) with an exhaust gas-driven charging device (7), comprising the following steps: - Providing a charge control variable (S) for a variably adjustable charge control variable (74) of the charging device (7) in order to set an efficiency of the charging device (7) with respect to an exhaust gas enthalpy of combustion exhaust gas provided by the internal combustion engine (2); - Determining a value for an exhaust back pressure limit (p 3lim ) based on a predetermined maximum ratio of charge exchange work (E A ) and a complete work (E T ) of the internal combustion engine (2) at a current operating point; - Limiting the turbocharger actuator size (S) depending on the specified exhaust backpressure limit value (p). 3lim ).

Description

Technical field

[0001] The invention relates to internal combustion engines with exhaust gas-driven charging devices, in particular methods for controlling a charging actuator of the charging device when a power increase of the internal combustion engine is requested. State of the art

[0002] To regulate boost pressure in turbocharged internal combustion engines, especially diesel engines, exhaust gas-driven charging devices, such as a turbocharger, are used. The efficiency of the charging device, or the proportion of mechanical power derived from the exhaust gas enthalpy that is used to drive a compressor, can be adjusted using a turbocharger actuator. Specifically, exhaust gas-driven charging devices can incorporate a controllable variable turbine geometry, a controllable wastegate valve, or similar devices to variably adjust the charging device's efficiency.

[0003] In turbocharged combustion engines, a significant change in load demand leads to a correspondingly abrupt change in the target boost pressure, which the compressor must then implement. To provide the target boost pressure as quickly as possible, a boost pressure control system activates the turbocharger actuator to rapidly build up exhaust back pressure, resulting in increased charge exchange losses. With a significant increase in load demand, it can happen that the power output or engine speed provided by the combustion engine initially drops before a power increase is achieved. This effect is called turbo lag.

[0004] Furthermore, during a large load change at low engine speeds, boost pressure build-up is the dominant factor, since the fuel quantity – and thus the torque – is limited at any given time along the smoke or soot limit solely by the air mass and therefore, in turn, by the boost pressure. Outside the smoke limit, boost pressure is not the limiting factor.

[0005] To optimize acceleration, the boost pressure build-up is limited at all times, ensuring that sufficient engine acceleration is always available, as engine start-up is also a key factor in boost pressure build-up. Therefore, the maximum duty cycle of the turbocharger's turbine is determined for each individual operating point through numerous driving tests. Each acceleration is characterized by a start and end point within the operating plane. The optimal trajectory through the operating plane must be found to achieve maximum acceleration.

[0006] German patent application DE 10 2009 010 633 A1 describes a method for reducing fuel consumption during transient operating conditions of a turbocharged engine. For this purpose, an intake air flow control device, such as guide vanes or a throttle valve, is set to a minimum closing limit to prevent an excessive increase in exhaust pressure and the associated pumping losses as soon as the current value of the mean effective pumping pressure (PMEP) exceeds a calibrated limit.

[0007] Publication GB 2491375 A discloses a method for controlling a turbocharged internal combustion engine with variable turbine geometry (VTG). To prevent engine damage, the boost pressure is limited by restricting the position of the turbocharger's guide vanes if a parameter value representing the pressure in the engine – such as the pump mean effective pressure (PMEP) or the exhaust manifold pressure – exceeds a predefined limit. Disclosure of the invention

[0008] According to the invention, a method for operating a turbocharged internal combustion engine according to claim 1, a device with the features of claim 8, an engine system with the features of claim 9, a computer program with the features of claim 10, and a machine-readable storage medium with the features of claim 11 are provided.

[0009] Further details are specified in the dependent claims.

[0010] According to a first aspect, a method for operating an internal combustion engine with an exhaust gas-driven charging device is provided, comprising the following steps: - Providing a charge controller size for a variable charge controller of the charging device in order to adjust the efficiency of the charging device with respect to the exhaust gas enthalpy of the combustion exhaust gas provided by the internal combustion engine; - Determining a specification for an exhaust backpressure limit value based on a given maximum ratio of charge exchange work and total work of the internal combustion engine at a current operating point; - Limiting the turbocharger control variable depending on the specified exhaust backpressure limit value.

[0011] A demand for increased power or load typically leads first to a corresponding increase in the target boost pressure in order to increase the boost pressure provided by the compressor. The target boost pressure is usually set using a boost pressure control system, which controls the actuator of the exhaust-driven turbocharging system via a suitable control variable to regulate the boost pressure.

[0012] By adjusting the turbocharger actuator to increase boost pressure, the exhaust back pressure is increased. This increased back pressure allows for greater mechanical power to be extracted from the turbocharger's turbine, which is used to compress the air and generate the increased boost pressure. In the case of a significant, especially sudden, increase in power or load, the turbocharger actuator is driven with a high control gradient to achieve a rapid increase in mechanical power, thus enabling a quick build-up of boost pressure.

[0013] On the one hand, a rapid build-up of boost pressure requires closing the turbocharger actuator as much as possible so that exhaust backpressure is available quickly. However, increased exhaust backpressure increases the combustion engine's gas exchange losses. This counteracts the engine's power increase, as the engine has to expel the exhaust gases against higher pressure. On the other hand, due to the dynamics of the air system, an increase in boost pressure does not directly translate into an increase in engine power. Therefore, increasing exhaust backpressure too quickly when increasing power can initially lead to a decrease in performance.

[0014] Since such a drop in the power supplied by the combustion engine should be avoided, a compromise must be found between the rates of boost pressure build-up and exhaust backpressure build-up (with the corresponding resulting power reduction) when an increase in power or load of the combustion engine is required. This compromise is currently achieved by applying a limit value for the change in the turbocharger actuator variable. This limit value is typically determined using a map based on engine speed and fuel injection quantity for the combustion engine.

[0015] The above procedure involves limiting the turbocharger control variable, as the control variable for the turbocharger actuator, when a power or load increase is requested, by specifying a maximum exhaust backpressure. The corresponding exhaust backpressure limit value is derived from physical quantities and depends only on application parameters characteristic of gas exchange losses.

[0016] This allows the limit to be defined based on the parameters relevant for increasing the load or accelerating the combustion engine. This simplifies the application of the limit to the turbocharger actuator control. Furthermore, limiting the turbocharger by using the exhaust pressure limit value can better reflect ambient conditions and different operating modes than directly limiting the turbocharger actuator value. If the exhaust gas-driven turbocharger model is adapted using additional sensors, optimal load increases or acceleration of the combustion engine can be achieved even when the turbocharger changes, particularly due to aging.

[0017] Furthermore, an actuator limit value can be determined depending on the exhaust back pressure limit value using a predefined throttle model or a predefined VTG turbine model, whereby the turbocharger actuator variable is limited by the turbocharger limit value.

[0018] Key parameters such as available total work and charge exchange work can be determined from the engine process. To achieve optimized acceleration, a limit value for the ratio of these parameters is determined or specified. Instead of the usual method of determining the turbocharger control variable limit based on operating state variables, the maximum exhaust back pressure is determined by the exhaust pressure limit value. From this, the maximum permissible turbocharger control variable can then be derived. The maximum exhaust back pressure can be understood as a reference parameter at the smoke limit.

[0019] It may be provided that the exhaust backpressure limit value is determined based on a ratio of static charge exchange work and total work at a momentary operating point assumed to be static, which is applied with a predetermined distribution parameter, in particular with a distribution factor.

[0020] In particular, the current operating point, which is assumed to be static, can be considered an operating point at which there is no change in the loader control variable.

[0021] According to one embodiment, the distribution factor can be specified depending on a rotational speed, a load, an instantaneous boost pressure control deviation of a boost pressure control that provides the turbocharger control variable.

[0022] Furthermore, the total work of a work cycle can be determined according to a Carnot process depending on a product of a gas mass in a stroke volume of the internal combustion engine at the beginning of the work stroke and a temperature difference between an exhaust gas temperature and a temperature of the intake air.

[0023] Alternatively, the total work of a work cycle can be determined via the amount of fuel injected in the work cycle and a thermal efficiency specified for the operating point of the internal combustion engine.

[0024] According to another aspect, a device, in particular a control unit, is provided for operating an internal combustion engine with an exhaust gas-driven charging device, wherein the device is designed to: - to provide a charge controller size for a variable charge controller of the charging device in order to adjust the efficiency of the charging device with respect to an exhaust gas enthalpy of combustion exhaust gas provided by the internal combustion engine; - to determine a specification for an exhaust backpressure limit value based on a given maximum ratio of charge exchange work and total work of the internal combustion engine at a current operating point; and - to limit the turbocharger control size depending on the specified exhaust back pressure limit value.

[0025] According to another aspect, an engine system with an internal combustion engine and an exhaust gas-driven charging device is provided, wherein the above device is provided to control a charge controller of the charging device according to the limited charge controller size.

[0026] According to another aspect, a computer program is provided which is set up to carry out all the steps of the above procedure. Brief description of the drawings

[0027] The embodiments are explained in more detail below with reference to the accompanying drawings. These show: Fig. 1 a schematic representation of an engine system with an internal combustion engine and an exhaust gas-driven charging device; Fig. 2. A flowchart illustrating a method for limiting the control of a charging actuator of the charging device; and Fig. 3 an energetic representation of an engine cycle process. Description of embodiments

[0028] In Fig. Figure 1 shows an engine system 1 with an internal combustion engine 2. The internal combustion engine 2 is designed as a four-stroke engine and can, in particular, be a diesel or gasoline engine. The internal combustion engine 2 has (for example, four) cylinders 3 to which air is supplied through an air supply section 4. A throttle valve 5 is arranged in the air supply section 4, with which the amount of air supplied to the cylinder 3 can be adjusted. Combustion exhaust gases are discharged from the internal combustion engine 2 via an exhaust gas discharge section 6.

[0029] To improve performance, the engine system 1 is equipped with an exhaust gas-driven charging device 7, which has a turbine 71 in the exhaust gas discharge section 6 and a compressor 72 in the air supply section 4. The turbine 71 is mechanically coupled to the compressor 72, for example via a shaft 73. The turbine 71 of the charging device 7 has a turbocharger actuator 74, with which the charging efficiency can be adjusted, i.e., the proportion of the available exhaust gas enthalpy that is converted into mechanical energy.

[0030] The turbocharger actuator 74 can be configured, for example, as a VTG actuator (VTG: Variable Turbine Geometry), a wastegate valve, or the like. During operation, the mechanical power provided by the turbocharger 7 is determined by the pressure difference between the inlet and outlet sides of the turbine 71, the exhaust gas mass flow rate through the turbine 71, and the position of the turbocharger actuator 74. The pressure on the inlet side of the turbine 71 is called exhaust back pressure, and on the outlet side, there is essentially air at ambient pressure or nearly ambient pressure if exhaust gas treatment devices, such as a catalytic converter, are installed downstream. The exhaust gas mass flow rate is essentially determined by the rotational speed of the internal combustion engine 2, i.e., by the amount of gas moved through the cylinders 3.

[0031] The compressor 72 draws in air from the environment, in particular via an air filter (not shown), and makes this available as charge air in a charge air section 41 on the outlet side of the compressor 72 between the compressor 72 and the throttle valve 5 at a charge pressure.

[0032] The operation of the internal combustion engine 2 is controlled by means of a control unit 10, which, based on a specified load requirement V and based on state variables such as rotational speed n and the like, controls the throttle valve 5 and the turbocharger actuator 74 in a suitable manner.

[0033] Starting from a specific exhaust back pressure p3 on the inlet side of the turbine 71, when there is an increased power or load demand, i.e., a demand for higher torque or higher rotational speed, the turbocharger actuator 74 is controlled so that a larger proportion of the supplied exhaust gas enthalpy is converted into mechanical power. Adjusting the turbocharger actuator 74 essentially increases the exhaust back pressure p3, thus increasing the pressure differential across the turbine 71. For a turbocharger actuator 74 designed as a variable turbine geometry, this is achieved by tilting or adjusting turbine blades. For a turbocharger actuator 74 designed as a wastegate valve located in a bypass line between the inlet and outlet sides of the turbine 71, this is achieved by reducing the flow cross-section in the bypass line.

[0034] The build-up of exhaust back pressure p3 initially increases the charge exchange losses of the combustion engine 2, before a change occurs due to the increasing boost pressure p LD Increasing the air supply to the combustion engine 2 increases the power output. This leads to a temporary drop in the power supplied by the combustion engine 2 during a rapid or sudden increase in power or load.

[0035] One way to avoid this effect is to limit the exhaust back pressure p3 and, for optimal load increase of the combustion engine 2, to find a compromise between the rate of charge pressure increase in the charge air section 41 (which leads to an increase in the power output of the combustion engine) or an increase in the intake manifold pressure p2 in an intake manifold section 42 located downstream of the throttle valve 5 and the exhaust back pressure p3 (the increase of which initially causes a reduction in power output). In particular, a smoke limit for the combustion engine 2 must also be taken into account.

[0036] The control unit 10 performs a variety of functions. In particular, the control unit 10 performs boost pressure control 11, which is based on a measured or modeled boost pressure p. LDIn the charge air section 41, it is now planned to limit a turbocharger control variable S. The turbocharger control variable S is usually defined as a duty cycle corresponding to a control variable value between 0 and 1.

[0037] The previous approach was to use a corresponding actuator limit value S. lim Determining the turbocharger control variable from a characteristic map that uses the injected fuel quantity and the engine speed as input variables requires considerable application effort. Furthermore, the necessary data is always dependent on the current operating conditions. These influences are either not considered or require significant additional application effort. Therefore, it is intended that the actuator limit value be selected in such a way that it is determined by physical quantities and depends only on application variables that are characteristic of charge exchange losses.

[0038] Determining the actuator limit value S lim The charge controller variable S is determined based on a maximum exhaust back pressure, which is derived from the respective operating state of the combustion engine.

[0039] For this purpose, key parameters such as available total work E are derived from the engine process. T and charge exchange work E A determined. The characteristic ratio r A as a ratio between total work E T and charge exchange work E A is determined by a predefined parameter limit value r Alim limited, so that an exhaust back pressure limit value p corresponding to the characteristic value limit value 3lim can be determined from the exhaust backpressure limit value p 3lim The actuator limit value S can be determined via a suitable throttle model or a VTG model (VTG: Variable Turbine Geometry). limThe limit value for the turbocharger control variable is determined. The exhaust backpressure limit value can be understood as a control variable at the smoke limit.

[0040] In conjunction with the flowchart of the Fig. 2 now describes a method for operating the internal combustion engine 2.

[0041] To optimize engine acceleration and boost pressure build-up, the work output of the engine cycle is determined. For this purpose, in Fig. 3 The engine cycle process is energetically represented as a diagram of the cylinder pressure p over a combustion chamber volume V, in particular a stroke volume V UT - V OT between a volume V OT at a top dead center of the cylinder motion and a volume V UT is depicted at the bottom dead center of the cylinder movement. The engine cycle includes the power stroke, which performs the total work E Ta single combustion provides, and the charge exchange cycle with the negative charge exchange work E A The entire work E T , the charge exchange work E A and the expansion losses E V are represented as areas in the diagram of Fig. 3 shown.

[0042] The engine work E M or the work delivered to the engine is determined to a first approximation as the difference in the total work E T of the work cycle and the charge exchange work E A , whereby expansion losses and other losses may be disregarded.

[0043] In step S1, the total work E T determined. The total work E TThe operating cycle can be estimated according to a Carnot process as the product of the gas mass m in the displacement volume at the beginning of the operating cycle and the temperature increase (i.e., the temperature difference T3 - T2 between the exhaust gas temperature T3 and the temperature T2 of the intake air) of this gas mass (taking into account the gas constant R). This holds true. ET≈m R(T3−T2)

[0044] Instead of the physical determination of the total work E T A work cycle via the Carnot process can be calculated as the total work E T It can also be determined via the energy input (in the form of the injection quantity) and the thermal efficiency at the operating point. This approach is particularly suitable for the control unit, as all the necessary parameters are already available.

[0045] In step S2, a charge exchange work E is performed. A determined. The charge exchange work E Ais described by the charge exchange cycle. The charge exchange work E can be used as an estimate. A It can be expressed as the product of the displacement volume V and the scavenging gradient, i.e., a pressure difference between the exhaust back pressure p3 and the intake manifold pressure p2. This applies. EA≈(p3−p2)V

[0046] In step S3, a boost pressure control is performed, which determines and provides a booster control variable S for the booster control unit 74 based on a specified target boost pressure.

[0047] In step S4, an exhaust backpressure limit value p is set for the current operating point. 3lim determined.

[0048] For the static operating case, in which there is no acceleration of the combustion engine 2, the turbine 71 of the charging device 7 is not accelerated. To maintain this operating point, the charge exchange work E required at the steady-state operating point is therefore AStatThe required turbine power is determined by this. Therefore, the characteristic ratio r can be used. A the charge exchange work E AStat to the overall work E T be determined (r A = E AStat / E T The ratio r M = 1 - r A This corresponds to the ratio of the engine work delivered to the crankshaft to the total work in steady-state operation. This simplified view is approximately valid for operating ranges up to medium vehicle speeds and thus relatively low steady-state losses (e.g., wind resistance, rolling resistance, etc.).

[0049] The following estimate applies to the relationship between compression work and total work: (p3−p2)VT≈rAET

[0050] If the driver requests acceleration and this requires an increase in boost pressure, this becomes apparent, among other things, through a boost pressure control deviation (difference between the target boost pressure and the measured or modeled actual boost pressure). To reduce the control deviation, a higher charge exchange work E is required. A as the stationary charge exchange work E AStat required. This increased charge exchange work E A or the increase in the charge exchange work ΔE A should be dimensioned so that the engine work E supplied to the combustion engine M sufficient to accelerate the combustion engine in an improved (optimized) manner. For this purpose, E TThe power is distributed in an applicable ratio (possibly also dependent on motor characteristics such as the current speed, load, and the like – here operating point op). This distribution is determined by the current steady-state distribution r. A and a constant or operating point-dependent distribution factor k as k* r A The distribution factor k can be specified depending on the engine speed, the load, and the instantaneous boost pressure control deviation. The distribution factor k represents the proportion of the total work E. T , which additionally increases the compression of the total work E available to the internal combustion engine 2 T is removed.

[0051] From the limitation for a limited performance ratio between EALim=k*rA*ET is calculated with EALim=(p3Lim−p2)V a maximum flushing gradient p 3lim -p2, so that p3lim≈(k*rAET) / V+p2 as exhaust back pressure limit value p 3lim results.

[0052] This allows the maximum exhaust backpressure limit value to be determined down to the distribution factor k. The distribution factor k can be predetermined and, in particular, should be between 1.05 and 1.3, preferably between 1.1 and 1.2.

[0053] From the exhaust back pressure limit value p 3lim In step S5, a controller limit value S is determined using a suitable throttle model or a VTG model (VTG: Variable Turbine Geometry). lim determined to limit the loader adjustment size S.

[0054] In step S6, the turbocharger control variable S provided by the boost pressure control is set to the actuator limit value S. lim limited and provided as a limited loader position size to the loader positioner 74.

[0055] Using calculation methods known from the literature for the total work E TFor a power stroke (describable exclusively by known combustion parameters such as mass in the combustion chamber, temperature rise, engine volume, compression ratio, and conversion efficiency), the only parameter required to determine the optimally limiting split ratio is the distribution factor k, in order to effectively quantify acceleration. The calculation of the turbocharger control variable limit thus becomes independent of the current engine speed, engine volume, and the associated mass flow rate.

Claims

[1] Method for operating an internal combustion engine (2) with an exhaust gas-driven charging device (7), comprising the following steps: - Providing a charge control variable (S) for a variably adjustable charge control variable (74) of the charging device (7) in order to set an efficiency of the charging device (7) with respect to an exhaust gas enthalpy of combustion exhaust gas provided by the internal combustion engine (2); - Determining a value for an exhaust back pressure limit (p 3lim ) based on a predetermined maximum ratio of charge exchange work (E A ) and a complete work (E T ) of the internal combustion engine (2) at a current operating point; - Limiting the turbocharger actuator size (S) depending on the specified exhaust backpressure limit value (p). 3lim ). [2] Method according to claim 1, wherein a controller limit value (S lim) depending on the exhaust back pressure limit value (p 3lim ) is determined using a predefined throttle model or a predefined VTG turbine model, where the turbocharger actuator variable (S) is limited by the actuator limit value (S lim ) is limited. [3] Method according to claim 1 or 2, wherein the exhaust back pressure limit value (p 3lim ) based on a ratio (r) applied with a given distribution size, in particular a distribution factor (k). A ) from a static charge exchange work (E AStat ) and a complete work (E T ) is determined at a momentary operating point assumed to be static. [4] Method according to claim 3, wherein the instantaneous operating point assumed to be static is assumed to be an operating point at which there is no change in the loader control variable (S). [5] Method according to claim 3 or 4, wherein the distribution factor (k) is predetermined depending on a rotational speed, a load, an instantaneous boost pressure control deviation of a boost pressure control which provides the turbocharger control variable (S). [6] Method according to any one of claims 1 to 5, wherein the total work (E T ) of a work cycle according to a Carnot process depending on a product of a gas mass (m) in a stroke volume of the internal combustion engine (2) at the beginning of the work stroke and a temperature difference between an exhaust gas temperature (T3) and a temperature (T2) of the intake air. [7] Method according to any one of claims 1 to 5, wherein the total work (E T ) of a work cycle via a quantity of fuel injected in the work cycle and a thermal efficiency specified for the operating point of the internal combustion engine (2). [8] Device, in particular control unit, for operating an internal combustion engine (2) with an exhaust gas-driven charging device (7), wherein the device is designed to: - to provide a charge control variable (S) for a variable charge control variable (74) of the charging device (7) in order to set an efficiency of the charging device with respect to an exhaust gas enthalpy of combustion exhaust gas provided by the internal combustion engine (2); - a specification for an exhaust back pressure limit value (p 3lim ) based on a predetermined maximum ratio of charge exchange work (E A ) and a complete work (E T ) of the internal combustion engine (2) at a current operating point; and - the turbocharger control variable (S) depending on the specified exhaust backpressure limit value (p 3lim to limit it. [9] Engine system with an internal combustion engine (2) and an exhaust gas-driven charging device (7), wherein a device according to claim 8 is provided to control a charging actuator (74) of the charging device (7) according to the charging actuator size limited by the exhaust back pressure limit value (S Lim ) to head towards. [10] Computer program which is configured to perform all steps of a method according to any one of claims 1 to 7. [11] Machine-readable storage medium on which a computer program according to claim 10 is stored. [12] Control unit comprising a machine-readable storage medium according to claim 11.

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