DE102024202377B3 - Method for controlling a combustion air ratio of an internal combustion engine and control device - Google Patents

Abstract

The invention relates to a method for controlling a combustion air ratio of an internal combustion engine (100), comprising the following steps: (a) determining a control response (121) to a control deviation (120) by means of a first controller (111), wherein the control deviation (120) corresponds to a deviation between an actual value (118) of a controlled variable (135) and a target value (119) of the controlled variable (135) and the controlled variable (135) is characteristic of the combustion air ratio; (b) converting the control response (121) by means of a manipulated variable (122) as an input of a controlled system (113); (c) receiving a change (143, 144) in the controlled variable (135) caused by the control response (121), which change is measured at or after an output of the controlled system (113) by means of a measuring element (117); (d) comparing the measured change (143, 144) of the controlled variable (135) with a model; and (e) changing, in particular reducing, or maintaining the control response (121) by means of a second controller (112) depending on the comparison. The invention further relates to a corresponding control device (104) and computer program.

Description

TECHNICAL FIELD

The present disclosure relates to methods for lambda control of an internal combustion engine and corresponding control devices which are configured to carry out such methods.

BACKGROUND OF THE INVENTION

During the incomplete combustion of an air-fuel mixture in an internal combustion engine, other gaseous combustion products are produced in addition to nitrogen, carbon dioxide and water. There are legal restrictions on the emission of hydrocarbons, carbon monoxide and nitrogen oxides, for example. When a three-way catalyst (TWC) is used in an internal combustion engine's exhaust system, these emissions are converted to nitrogen, carbon dioxide and water.

However, such a conversion is only optimal at a stoichiometric operating point, which is characterized by a combustion air ratio lambda equal to one. To achieve this operating point, a lambda control is carried out in an engine control unit of the combustion engine. The lambda control is typically based on the signals from lambda sensors before and after the three-way catalytic converter. For the lambda control before the three-way catalytic converter, the so-called pre-catalytic converter control, the oxygen content of the exhaust gas before the catalytic converter is measured using a linear lambda sensor. Depending on the measured value, the control corrects the amount of fuel injected from the pre-control of the combustion engine.

The accuracy can be further improved with the signal from the second lambda sensor behind the catalytic converter.

The quality of the pre-catalyst control plays an important role in the accuracy, speed and stability of the lambda control. These control properties in turn have a direct influence on emissions, because any deviation from a specified lambda target value or lambda setpoint causes emissions. Optimal lambda control is therefore particularly important in view of new, increasingly strict emissions legislation.

PI controllers have often been used for pre-catalyst control, but these have stability problems under certain operating conditions. Such stability problems can, for example, lead to a constant or systematic deviation from the lambda target value, which also worsens the pollutant emissions of the combustion engine. In addition, the engine can become unstable, which can affect the acoustics of the combustion engine or the driving characteristics of the vehicle. In the worst case, uncontrolled oscillations can cause considerable damage to the drive system or the entire vehicle.

For the reasons mentioned above, robust controllers with a slow control response are now mostly used for lambda control. However, this is at the expense of an optimal control response and generally leads to higher emissions.

Through document DE 10 2006 034 685 A1 A method and a device for controlling the fuel-air mixture of an internal combustion engine are disclosed. The aim here is to improve the potential of a robustly designed controller with regard to the effectiveness of the aftertreatment of the exhaust gas. To do this, a control deviation between a setpoint and an actual value of the controlled variable measured with an oxygen sensor is first determined, and a control signal is generated at the controller output. In addition, the controlled variable and the control signal are fed to a controlled system identification, on the basis of which the controller parameters are adjusted.

document DE 102 59 312 A1 describes a control device for the air-fuel ratio of an internal combustion engine and a corresponding method. During control, an actual value is determined using a lambda sensor, this is compared with a target value and this control deviation is processed in a controller to form a controller output signal for the injection quantity of the internal combustion engine. In addition, the actual value of the controlled variable and the controller output signal are fed to a model which changes the input signal of the controller.

BRIEF DESCRIPTION AND EMBODIMENTS

It is therefore an object of the present disclosure to provide a method for controlling a combustion air ratio of an internal combustion engine and a corresponding control device which enable fast, precise, but at the same time robust control, in particular in order to reduce emissions of the internal combustion engine.

This object is achieved by a method and a control device according to the independent patent claims. Advantageous embodiment Further developments and refinements arise from the respective dependent claims, the following description and the drawings.

Thus, according to a first aspect, a method for controlling a combustion air ratio of an internal combustion engine is provided. The method has the following steps: (a) determining a control response to a control deviation by means of a first controller, wherein the control deviation corresponds to a deviation, in particular a difference, between an actual value of a controlled variable and a target value of the controlled variable and the controlled variable is characteristic of a combustion air ratio; (b) implementing the control response by means of a manipulated variable as an input of a controlled system; (c) receiving a change in the controlled variable caused by the control response, which is measured at or after an output of the controlled system by means of a measuring element; (d) comparing the measured change in the controlled variable with a model; and (e) changing, in particular reducing, or maintaining the control response by means of a second controller depending on the comparison.

According to a further aspect, a control device with a first controller and a second controller is provided, which is configured to carry out the method described above.

According to a further aspect, a computer program is provided which comprises instructions which, when the computer program is executed by a computer, cause the computer to carry out the method described above.

In the context of the present disclosure, a controlled system is defined, for example, as that part of a control loop that contains the system or process to be controlled. The controller(s) of the control loop can be set up to influence the controlled system via one or more manipulated variables. If the manipulated variable characterizes a fuel injection into an internal combustion engine and the controlled variable characterizes a combustion air ratio or lambda, the controlled system can, for example, describe the physical relationship between fuel injection into the internal combustion engine and measurement of the combustion air ratio in an exhaust system downstream of the internal combustion engine.

In the context of the present disclosure, a manipulated variable is defined, for example, as an output variable of a controller and/or as an input variable of the controlled system. The manipulated variable can be suitable for influencing the controlled variable. A control response of the controller can be implemented by means of the manipulated variable. For example, the manipulated variable can characterize a fuel injection.

In the context of the present disclosure, a controlled variable is defined, for example, as an output variable of the controlled system. It can be a variable to be controlled, which is to be controlled, for example, in such a way that it corresponds to a predetermined target value or target curve. The controlled variable can be, for example, a combustion air ratio or lambda. The combustion air ratio can be or characterize the ratio of air to fuel in comparison to a combustion stoichiometric mixture. The combustion stoichiometric mixture can be an optimal ratio between the reactants, for example combustible substances and oxygen as an oxidizing agent.

In the context of the present disclosure, a measuring element is defined, for example, as a measuring device at or after the output of the controlled system, which is in particular set up to determine the controlled variable directly or indirectly. The measuring element can be set up to determine measured values from which corresponding values of the controlled variable can be derived. The measuring device can belong entirely or partially to the controlled system or can be designed separately from the controlled system. For example, the measuring element can be a measuring device which is set up to determine an oxygen content in an exhaust gas of the internal combustion engine, from which the combustion air ratio can be derived at least approximately.

The device and/or method described above may be advantageous to enable fast, accurate and at the same time robust control.

In particular, if there are inaccuracies in the model parameters for the first controller or if there are disturbances in the control system and/or the measuring element, for example in the case of an aging lambda sensor or when replacing the lambda sensor, a first controller designed for fast or optimal control can have stability problems. The greater the inaccuracy or the disturbance, the less stable the control loop can be. As a result, in the event of instability, the adjustment of the control loop can be impaired to such an extent that an actual control value shows a constant deviation from the target control value or target control curve. This can in turn lead to increased pollutant emissions.

To counteract such stability problems, the measured change in the controlled variable is compared with a model, for example a Model of the controlled system and/or the measuring element, whereupon a second controller changes the control response, in particular reduces it, or maintains it depending on the comparison. In other words, the model deviations determined by the comparison are an indication of possible instabilities in the control. If such model deviations are determined, the control response can be changed accordingly, in particular reduced. For example, the control response can be slowed down or its amplitude reduced. This can increase the robustness of the control when stability problems occur.

This not only reduces pollutant emissions, but also stabilizes the engine under a wide range of operating conditions, thereby improving driving acoustics or overall driving characteristics. Furthermore, harmful oscillations in the drive system or vehicle can be avoided or at least reduced in terms of system component protection.

According to one embodiment, the model is or comprises a model of the controlled system and/or the measuring element. Only aspects of the controlled system and/or the measuring element that are relevant to the controlled variable can be modeled. It can therefore be a very abstract model that, for example, only includes a few mathematical formulas, for example n-order polynomial functions, and/or one or more tables. The model can, for example, be an n-order system with dead time, in particular a first-order system with dead time. The first controller can determine the control response based on the model.

According to one embodiment, the deviation between the actual value of the controlled variable and the target value of the controlled variable corresponds to a difference. Other functional dependencies are also possible.

According to one embodiment, the manipulated variable is characteristic of a quantity of fuel injected into the internal combustion engine. Such an embodiment can be advantageous because the quantity of fuel injected represents a typical manipulated variable when controlling the combustion air ratio.

According to one embodiment, the measuring element is a lambda probe which measures an oxygen content in an exhaust gas of the internal combustion engine. Such lambda probes for indirectly measuring the combustion air ratio are typically used in lambda control.

According to one embodiment, the lambda probe is arranged in an exhaust system of the internal combustion engine upstream of a catalytic converter, in particular a three-way catalytic converter. The lambda probe is therefore arranged between the internal combustion engine and the catalytic converter. In other words, the associated lambda control is a so-called pre-catalytic converter control. This can be advantageous for low-emission operation of the catalytic converter.

According to one embodiment, model parameters of the model are determined at least partially by diagnosing the lambda probe, i.e. the lambda probe actually used in the exhaust system, or a nominal lambda probe. A production tolerance can be taken into account for the nominal lambda probe. Such a calibration of the model can be advantageous in order to avoid costly re-diagnoses for model adaptation.

According to one embodiment, the second controller is calibrated based on nominal plant parameters. Such an embodiment can be advantageous to avoid further parameter identification and model adaptation. In addition, no new diagnosis is required to monitor the adaptation. The existing diagnosis of the first controller for minimum/maximum plausible point intervention is still valid. According to one embodiment, the automatic adjustment of the control strategy between optimality and robustness is based only on the uncertainty of the model.

According to one embodiment, the model comprises a model of the controlled system and/or the measuring element and comprises comparing: modeling a comparison control response by applying a model that is inverse to the model to the measured change in the controlled variable, and comparing the comparison control response with the control response. Such an embodiment can be advantageous in order to quantify any deviations from the model and to effectively compensate for possible control instabilities.

According to one embodiment, the model comprises a model of the controlled system and/or the measuring element and comprises comparing: modeling a comparison change of the controlled variable using the model of the controlled system depending on the control response and comparing the comparison change of the controlled variable with the measured change of the controlled variable. Such an embodiment can be advantageous in order to detect any deviations quantify the model's deviations and effectively compensate for possible control instabilities.

According to one embodiment, a dead time determined by the controlled system is taken into account during the comparison, in particular by a dead time element of the second controller. Such a dead time can be modeled either by the model or separately from the model. In the latter case, a temporal progression would not be taken into account in the model and the modeling of the control intervention would be time-synchronous. Such an embodiment can be advantageous in order to be able to assign changes in the manipulated variable and corresponding effects on the controlled variable in time.

According to one embodiment, the dead time corresponds to or correlates with a running time from the injection of the fuel to the measurement of the associated lambda value by the measuring element.

According to one embodiment, a measure for a deviation of the measured change in the controlled variable from the model is determined during the comparison and the change, in particular the reduction, of the control response takes place depending on the measure. According to one embodiment, the control response is reduced more the greater the deviation determined by the measure. The dependency between the measure and the reduction in the control response can be monotonic or strictly monotonic. Such an embodiment can be advantageous because larger deviations require a stronger reaction in order to avoid instabilities in the control.

According to one embodiment, the changing or maintaining of the control response is determined by a filter element which characterizes a sensitivity and/or a speed, in particular with respect to the changing or maintaining of the control response. According to one embodiment, the filter element has a low-pass filter.

According to one embodiment, the measure is characteristic of a difference between the control response implemented by means of the manipulated variable and the comparison control response and/or of a difference between the measured change in the controlled variable and the comparison change in the controlled variable.

According to one embodiment, the measure is indicative of an inaccuracy of one or more model parameters of the model and/or of a disturbance variable of the controlled system that is at least not fully taken into account by the model. In the context of the present disclosure, a disturbance variable is defined, for example, as a variable acting on the controlled system and/or the measuring element that is capable of influencing the controlled variable. An inaccurate model parameter can, for example, result from aging or replacement of the measuring element, such as a lambda probe. A disturbance variable can, for example, be external damage to the measuring element that causes a pulsating signal from the lambda probe.

According to one embodiment, the second controller is configured to increase the robustness of the control depending on the measure. In other words, the first controller aims at an "optimal", in particular fast, control, while the second controller is configured to correct the first controller with a view to a "robust", in particular stable, control under many operating conditions.

According to one embodiment, the control response of the first controller is based on the model, in particular the model of the controlled system and/or the measuring element. Such an embodiment can be advantageous because the second controller changes the control response in particular when the control response of the first controller is based on incorrect model assumptions. In particular under such incorrect model assumptions, instabilities in the control are to be expected.

According to one embodiment, the first controller is configured such that a rise time, an overshoot, a settling time and a state error or stationary error are taken into account for the control response. The rise time can be preferred over the other parameters. The overshoot can be preferred over the remaining parameters, i.e. except rise time. Such an embodiment can characterize an "optimal" control.

According to one embodiment, the second controller is designed to carry out at least one of the following measures for changing, in particular reducing, the control response: increasing a rise time; reducing an overshoot; reducing a state error or stationary error. The rise time, the overshoot and/or the stationary error can relate to the measured change in the controlled variable. Such an embodiment can be advantageous because the measures mentioned all contribute to the robustness of the control.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and advantageous embodiments and further developments of the method and the control unit result from the following, in connection with the figures illustrated embodiments.

• 1 eine Brennkraftmaschine mit einem Abgasstrang und einem Steuergerät gemäß einer Ausführungsform der vorliegenden Offenbarung;
• 2 und 3 jeweils Regelkreise für Verfahren zur Regelung eines Verbrennungsluftverhältnisses gemäß Ausführungsformen der vorliegenden Offenbarung;
• 4 bis 6 verschiedene Aspekte eines Vergleichs zwischen optimaler und robuster Regelung bei Verfahren zur Regelung eines Verbrennungsluftverhältnisses gemäß Ausführungsformen der vorliegenden Offenbarung.

They show:

• 1 an internal combustion engine with an exhaust system and a control unit according to an embodiment of the present disclosure;
• 2 and 3 each show control loops for methods for controlling a combustion air ratio according to embodiments of the present disclosure;
• 4 to 6 various aspects of a comparison between optimal and robust control in methods for controlling a combustion air ratio according to embodiments of the present disclosure.

Elements that are the same, similar or have the same effect are given the same reference symbols in the figures. In some figures, individual reference symbols have been omitted to improve clarity. The figures and the proportions of the elements shown in the figures to one another are not to be regarded as being to scale. Rather, individual elements may be shown exaggeratedly large for better representation and/or for better comprehensibility.

DETAILED DESCRIPTION OF EMBODIMENTS

1 shows an internal combustion engine or combustion engine 100 and an exhaust system 101 coupled to the internal combustion engine 100. The exhaust system 101 comprises a three-way catalytic converter 103 and a lambda probe 102 downstream and upstream of the catalytic converter 103. The lambda probe 102 arranged between the internal combustion engine 100 and the three-way catalytic converter 103 is a linear lambda probe which is designed to measure the oxygen content of the exhaust gas upstream of the three-way catalytic converter 103. On the basis of this measured value, a control unit 104 carries out the lambda control upstream of the three-way catalytic converter 103, i.e. the so-called pre-catalyst control. Depending on the measured value, the control unit 104 corrects the control of the fuel quantity from the pre-control of the internal combustion engine 100. For greater accuracy, the signal from the second lambda probe 102 downstream of the three-way catalyst 103 is also taken into account.

2 and 3 show circuit diagrams or control circuits 110 of processes that are used by the 1 shown control unit 104 for pre-cat control. In 2 The pre-cat control is shown as a cascade control with two parts or two controllers. The first controller 111 is an optimal controller and the second controller 112 connected downstream of the first controller 111 is a robust controller. The control response of the second controller is used as the input of the controlled system 113. A controlled variable at the output of the controlled system 113 is fed back to the second controller 112 on the one hand and compared with a reference variable on the other hand, whereby the deviation of the controlled variable from the reference variable serves as the input of the first controller 111.

The optimal controller 111 is a controller that meets defined criteria in terms of control quality. This can be, for example, an IMC (Internal Model Regulator), MPC (Model Predictive Regulator), LQR (linear quadratic regulator) or a PI controller, in particular an optimally calibrated PI controller. Control quality refers, among other things, to the signal metrics rise time, percent overshoot, settling time and steady state error, which characterize a deviation from the lambda target value and can therefore also influence emissions.

In 3 is compared with 2 the robust controller 112 is divided into several components, including the inverse model 114, the dead time element 115, the filter 116, the comparison element 124 and the reduction element 125. The controlled system 113 can be described as an n-th order system with dead time, in particular as a first order system with dead time. An inaccuracy in the parameters of the model or an unknown disturbance caused by a disturbance variable 123 in the controlled system 113 can be referred to as an indeterminacy of the system.

The robust controller compensates for this uncertainty at least partially. To do this, the inverse model 114 reconstructs the control intervention from the lambda signal measured by the measuring element 117. In the control loop 110 shown, the modeling by the inverse model 114 is time-synchronous. However, because the controlled system 113 contains a time delay between input and output, this delay must be compensated by the dead time element 115. At the comparison element 124, a delayed input of the controlled system 113 is therefore compared with the control intervention reconstructed from the output of the controlled system.

The filter 116 defines the dynamics and sensitivity of the compensation, depending on the deviation between the actual and the reconstructed control intervention determined by the comparison element 124. For example, a low-pass filter can be used as filter 116. In a In the ideal case, when the uncertainty is zero, the robust controller does not provide compensation via the reduction element 125 because there is no deviation between the reconstructed control intervention and the delayed active control intervention at the comparison element 124. The control loop behaves optimally to a certain extent. However, if the uncertainty increases, the robust controller compensates the output of the first controller 111 or the optimal controller so that the control loop 110 remains stable.

The first controller 111 receives an actual value 118 of the controlled variable determined by means of the measuring element 117 and a setpoint value 119 of the reference variable. From the comparison of the actual value 118 with the setpoint value 119, a control deviation 120 is formed as the input of the first controller 111. In response to the control deviation 120, the first controller 111 determines a first control response 121, which is then changed or maintained depending on the model deviation determined at the reduction element 125 and the compensation determined therefrom by the filter 116, in order to form a second control response 121 of the second controller. The second control response 121 is implemented by means of a manipulated variable 122 as the input of the controlled system 123 in order to obtain an actual value 118 of the controlled variable as the output of the controlled system 123, which is measured by means of the measuring element 117. The combination of the first optimal controller 111 and the second robust controller enables precise and stable control of the actual value 118 to the setpoint 119.

4 shows the reaction over time 130 of the actual value 118 of the controlled variable 135 to a sudden change in the setpoint 119 of the reference variable from a smaller constant value to a larger constant value. After the change in the setpoint 119, the actual value 118 changes in the direction of the changed setpoint 119. The time until the changed setpoint is reached is characterized by the rise time 131. The rise time 131 can be defined based on a tangent to the course of the actual value 118 at the intersection with the changed setpoint 119. An overshoot of the actual value 118 after the intersection with the changed setpoint 119 is characterized by the overshoot 133. The overshoot 133 can be defined as the largest amplitude of a deviation from the changed setpoint 119 or from a steady-state value of the actual value 118 after the intersection. After one or more oscillations of the actual value 118 around the changed setpoint 119, the actual value 118 approaches the setpoint 119. The total time of this approach starting from the time of the change of the setpoint 119 is characterized by the settling time 132. Possible deviations of the actual value 118 from the setpoint 119 after the settling time 132 are referred to as stationary error 134 or state error.

5 shows a control intervention in the case of uncertainty. The robust controller reduces the control response 141 of the optimal controller to the robust control response 142. As a result, as in 6 As shown, the control is adapted, for example, in the event of a sudden change in the setpoint 119, from an optimal control 143 with a fast rise time to a robust control 144 with a slow rise time, but a small overshoot and a small stationary error.

The invention is not limited to the embodiments by the description thereof. Rather, the invention encompasses any new feature and any combination of features, which in particular includes any combination of features in the embodiments and patent claims.

REFERENCE SIGN

100

internal combustion engine

101

exhaust system

102

lambda sensor

103

catalyst

104

control unit

110

control loop

111

first controller

112

second controller

113

controlled system

114

inverse model

115

dead time element

116

filter

117

measuring element / lambda probe

118

actual value of the controlled variable

119

target value of the controlled variable / reference variable

120

control deviation

121

rule response

122

manipulated variable

123

disturbance

124

comparison element

125

reduction element

130

Time

131

rise time

132

settling time

133

overshoot

134

stationary error

135

controlled variable

141

control response of the first controller

142

control response of the second controller

143

Controlled variable to control response of the first controller

144

controlled variable to control response of the second controller

Claims (15)

Method for controlling a combustion air ratio of an internal combustion engine (100), comprising the following steps: - Determining a control response (121) to a control deviation (120) by means of a first controller (111), wherein the control deviation (120) corresponds to a deviation between an actual value (118) of a controlled variable (135) and a target value (119) of the controlled variable (135) and the controlled variable (135) is characteristic of the combustion air ratio; - Implementing the control response (121) by means of a manipulated variable (122) as an input of a controlled system (113); - Receiving a change (143, 144) in the controlled variable (135) caused by the control response (121), which is measured at or after an output of the controlled system (113) by means of a measuring element (117); - comparing the measured change (143, 144) of the controlled variable (135) with a model; and - changing, in particular reducing, or maintaining the control response (121) by means of a second controller (112) depending on the comparison. Method according to the preceding claim, wherein the manipulated variable (122) is characteristic of a quantity of fuel injected into the internal combustion engine (100). Method according to one of the preceding claims, wherein the measuring element (117) is a lambda probe (117) which measures an oxygen content in an exhaust gas of the internal combustion engine (100). Method according to the preceding claim, wherein the lambda probe (117) is arranged in an exhaust system (101) of the internal combustion engine (100) upstream of a catalyst (103), in particular a three-way catalyst. procedure according to claim 3 or 4 , wherein model parameters of the model are determined at least partially by a diagnosis of the lambda probe (117) or a nominal lambda probe. Method according to one of the preceding claims, wherein the model comprises a model of the controlled system (113) and/or the measuring element (117) and the comparing comprises: modelling a comparison control response by applying a model (114) inverse to the model to the measured change (143, 144) of the controlled variable (135), and comparing the comparison control response with the control response (121). Procedure according to one of the Claims 1 until 5 , wherein the model comprises a model of the controlled system (113) and/or the measuring element (117) and the comparison comprises: modeling a comparison change of the controlled variable (135) by means of the model depending on the control response (121) and comparing the comparison change of the controlled variable (135) with the measured change (143, 144) of the controlled variable (135). procedure according to claim 6 or 7 , wherein in the comparison a dead time determined by the controlled system (113) is taken into account, in particular by a dead time element (115) of the second controller (112). Method according to one of the preceding claims, wherein during the comparison a measure for a deviation of the measured change (143, 144) of the controlled variable (135) from the model is determined and the changing, in particular the reducing, of the control response (121) takes place as a function of the measure. Method according to the preceding claim, if related to one of the Claims 6 until 8 , wherein the measure is characteristic of a difference between the control response (121) implemented by means of the manipulated variable (122) and the comparison control response and/or of a difference between the measured change (143, 144) of the controlled variable (135) and the comparison change of the controlled variable (135). procedure according to claim 9 or 10 , wherein the measure is indicative of an inaccuracy of one or more model parameters of the model and/or of a disturbance variable (123) of the controlled system (113) which is at least not fully taken into account by the model. Method according to one of the preceding claims, wherein the control response (121) of the first controller (111) is based on the model. Method according to one of the preceding claims, wherein the second controller (112) is a is directed to carry out at least one of the following measures for changing, in particular reducing, the control response (121): increasing a rise time (131); reducing an overshoot (133); reducing a state error or stationary error (134). Control device (104) with a first controller (111) and a second controller (112), which is configured to carry out the method according to one of the preceding claims. Computer program which comprises instructions which, when the computer program is executed by a computer, cause the computer to carry out the method according to one of the Claims 1 until 13 to carry out.

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