EP2786002B1 - Method for operating a combustion engine and device for implementing the method - Google Patents

Description

The invention relates to a method for operating an internal combustion engine, wherein an exhaust gas generated by the internal combustion engine is guided over a arranged in an exhaust passage 3-way catalyst.

State of the art and technological background

Processes for lambda control in internal combustion engines can be used to reduce the emissions of harmful exhaust gases into the environment. For this purpose, at least one catalyst can be arranged in the exhaust system of the internal combustion engine. In order to keep the catalyst at an optimum operating point, it is necessary to control the mixture preparation of the internal combustion engine with the aid of a lambda control in such a way that, at least in the mean value, a regulated lambda value is obtained which is as close as possible to 1.0. To generate a measurement signal, a lambda probe can be arranged in the exhaust system of the internal combustion engine.

The state of the art is inter alia the application of one of the two control methods described below.

In FIG. 2 illustrated is a control method, as is commonly used when using jump lambda probes. The upper curve shows the probe signal versus time and the lower curve the regulator action against time. These probes change the direction of the controller when the probe crosses a predetermined threshold, for example 450mV, which is the stoichiometric point (here at times t1, t2 and t3). The course of the signal above or below the respective threshold is not further used or utilized in the regulation, but the adjustment takes place independently of this pilot-controlled, usually via a fixed P and I share, which in turn depends on other variables such as for example, the operating point.

A disadvantage of this method is the comparatively slow control speed, since above or below the control threshold, the absolute signal value no further is considered and thus larger mixture deviations are compensated only with the previously determined control speed. Furthermore, it is disadvantageous that the switching frequency is comparatively high and is essentially determined only by the line transit time to the probe and the probe dead time. Thus, it is not possible to predetermine the oxygen introduction or discharge into the downstream catalyst, so that the conversion efficiency of the catalyst is limited.

In FIG. 3 is a control method, as is usually the case when using probes with accurate lambda signal also outside the stoichiometric point, so usually broadband lambda probes is applied (lambda actual value from the probe signal: bold solid curve, Lambda setpoint at the probe: narrow solid curve, control variable: bold dashed curve, engine lambda nominal value: narrow dashed square curve). The modulation is set via a variation of the lambda setpoint. From the difference between the desired value and the measured actual value, the control deviation is determined, which is supplied to a suitable controller (for example PID controller). A consideration of the track behavior takes place, if not the motor target value is used for calculating the difference, but, taking into account the track runtime, the course of the motor target value is related to the position of the probe and this value is used as setpoint at the probe position.

An advantage of this method is that the desired lambda value can be set accurately and the controller has a fast control speed. The disadvantage is that it can lead to overshoot of the controller and stronger fluctuations of the fuel-air mixture, if the stored track behavior does not match the actual line dynamics. This is the case, for example, when the probe becomes more sluggish due to aging or poisoning. This is exemplary in FIG. 4 displayed (Lambda actual value from the probe signal: rich dark curve, control value of the controller: bold bright curve, engine lambda nominal value: narrow rectangle curve). The probe signal is significantly slower here than in FIG. 3 , At the time t1, when the probe signal reaches the setpoint, therefore, the controller value is already strong and it comes in the sequence to overshoot in the controller and the lambda value (time t2), and only with delay, the setpoint can be stabilized stable (time t3 ). This is detrimental to the efficiency of the downstream catalyst, that is, it comes to increased emissions, with larger fluctuations in the fuel-air ratio, this can also cause a noticeable jerking of the engine.

If the lambda signal is determined from the signal of a leap lambda probe, a regulator has FIG. 3 yet another disadvantage. In FIG. 5 is a typical characteristic of jump lambda probes shown. The jump range, that is to say the range of the large signal change, can be recognized in the range of lambda = 1. Today's probes dynamically react in this jump range slower than in the purely fat or purely lean range. A lambda signal calculated from a jump-type signal therefore has a time delay in the case of a mixture change between rich and lean exhaust gas at the lambda = 1 range. This is in FIG. 4 to recognize at time t4. In the case of this controller type, this behavior also leads to overshoots in the controller value and, as a result, in the lambda value, as shown at time t5, with the disadvantages described above. Alternatively, the control parameters could be adapted to the reduced dynamics in the lambda = 1 point, but then the controller would be much slower in the range outside the lambda = 1 range than it could actually be.

Out DE 10 2006 049 656 A1 An approach is already known, as with probes with inaccurate correlation between signal and actual mixture composition in the range outside the stoichiometric point (ie, for example, jump probes), which in the prior art, the in FIG. 2 illustrated method is applied, benefits of in FIG. 3 illustrated method can be developed. It describes how a changeover of the controller direction takes place only when not only a signal threshold value is exceeded or not exceeded, but additionally also a threshold value for a variable derived from the probe signal. This can be ensured with a certain accuracy for a defined oxygen input or discharge into the catalyst and thus the conversion efficiency of the catalyst can be increased. However, there remains the disadvantage of the slow control of mixture deviations.

Summary of the invention

One or more of the mentioned problems of the prior art can be eliminated or at least reduced with the aid of the method according to the invention for operating an internal combustion engine. According to the method, an exhaust gas generated by the internal combustion engine is passed over a 3-way catalyst arranged in the exhaust passage. A lambda probe detects a variable characteristic of an exhaust lambda in front of the 3-way catalytic converter and forwards it to an engine control unit with integrated PI or PID controller. By setting a setpoint value, the PI or PID controller of the engine control unit sets a substantially stoichiometric exhaust lambda and the exhaust lambda with predetermined periodic setpoint variation alternately deflected in the direction of a lean lambda value and a lambda lambda value (lambda modulation). At the beginning of each setpoint variation, a pre-controlled P-component with subsequent I-component is preset up to a time t2, the time t2 being set by means of stored parameter characterizing a time-lapse behavior such that at this time t2 the probe signal or a variable derived therefrom would have reached the setpoint specification. From the point in time t2, switching is effected to a regulation for a predefinable period of time until the end of the respective desired value variation, which is based on a difference between an actual value and the desired value of the lambda probe or a variable derived therefrom.

The invention is based on the finding that a change from the pilot-controlled regulator setting to a (preferably continuous) control brings with it the advantages of the two different controller types without the disadvantages of the two controller types having to be accepted.

Preferably, a size of the P component is determined as a function of a desired amplitude of the setpoint variation. An I component can then be set so that at time t2 the probe signal or a quantity derived therefrom would reach the setpoint.

A preferred variant of the method provides that for determining a reaction time of the lambda probe, a minimum reaction of the lambda probe is defined in comparison to the state before the controller switching and the time is detected as the reaction time, which has passed since the controller switching to the minimum response of the lambda probe. However, the response time is preferably only determined if the setpoint specified by the PI or PID controller exceeds a predetermined minimum size. The reaction time can be recorded separately from the lambda probe after a fat-lean jump and a lean-fat jump.

Another aspect of the present invention relates to a control device for controlling an operation of an internal combustion engine, which is set up for carrying out the method according to the invention. For this purpose, the controller may include a computer-readable control algorithm for performing the method. In an advantageous embodiment, the control unit is an integral part of the engine control unit.

Further preferred embodiments of the invention will become apparent from the remaining, mentioned in the dependent claims characteristics or from the following description.

Figur 1

schematischer Aufbau einer Verbrennungskraftmaschine mit einer Abgasanlage und 3-Wege-Katalysator;

Figur 2

zeitlicher Verlauf des Abgaslambdas stromauf des 3-Wege-Katalysators sowie des Reglereingriffs nach einer ersten Variante des herkömmlichen Verfahrens;

Figur 3

zeitlicher Verlauf des Abgaslambdas stromauf des 3-Wege-Katalysators sowie des Reglereingriffs nach einer zweiten Variante des herkömmlichen Verfahrens;

Figur 4

Verhalten des Reglers für das herkömmliche Verfahren gemäß Fig. 3 bei unpassenden Streckenparametern;

Figur 5

Kennlinie einer Sprung-Lambdasonde für das herkömmliche Verfahren gemäß Fig. 3;

Figur 6

zeitlicher Verlauf des Abgaslambdas stromauf des 3-Wege-Katalysators sowie des Reglereingriffs nach dem erfindungsgemäßen Verfahren; und

Figur 7

Ermittlung der Sprungantwortzeit nach dem erfindungsgemäßen Verfahren.

The invention will be explained below in embodiments with reference to the accompanying drawings. Show it:

FIG. 1

schematic structure of an internal combustion engine with an exhaust system and 3-way catalytic converter;

FIG. 2

time course of the exhaust gas lambda upstream of the 3-way catalyst and the regulator engagement according to a first variant of the conventional method;

FIG. 3

time course of the exhaust gas lambda upstream of the 3-way catalyst and the regulator engagement according to a second variant of the conventional method;

FIG. 4

Behavior of the controller for the conventional method according to Fig. 3 with inappropriate route parameters;

FIG. 5

Characteristic curve of a jump lambda probe for the conventional method according to Fig. 3 ;

FIG. 6

time course of the exhaust gas lambda upstream of the 3-way catalyst and the regulator intervention according to the inventive method; and

FIG. 7

Determining the step response time according to the method of the invention.

FIG. 1 schematically shows the structure of an internal combustion engine 10 with a downstream exhaust system. The internal combustion engine 10 may be a spark ignition engine (gasoline engine). With regard to their fuel supply, they can have a direct injection fuel supply, so working with internal mixture formation, or have a pilot fuel injection and thus work with external mixture formation. In addition, the internal combustion engine 10 can be operated homogeneously, with a homogeneous air-fuel mixture present at the ignition point in the entire combustion chamber of a cylinder, or in an inhomogeneous mode (stratified charge mode) in which a comparatively rich air-fuel mixture at the time of ignition, especially in the area of a spark plug, is present, which is surrounded by a very lean mixture in the remaining combustion chamber. Important in the context of the present invention is that the internal combustion engine 10 with a substantially stoichiometric air-fuel mixture can be operated, that is, with a mixture with a lambda value close to or equal to 1.

The exhaust system comprises an exhaust manifold, which merges the exhaust gas of the individual cylinders of the internal combustion engine 10 into an exhaust gas channel 16. In the exhaust passage 16, various exhaust gas purifying components may be present. Essential within the scope of the present invention is a 3-way catalyst 20 arranged in the exhaust gas duct 16.

The 3-way catalyst 20 has a coating of catalytically active components, such as platinum, rhodium and / or palladium, which are on a porous catalyst support, for example, Al ₂ O ₃ , applied. The coating further comprises an oxygen storage component, for example cerium oxide (CeO ₂ ) and / or zirconium oxide (ZrO ₂ ), which determines the oxygen storage capacity (OSC) of the 3-way catalyst 20. In a stoichiometric or slightly rich exhaust gas atmosphere, the 3-way catalyst 20 can reduce nitrogen oxides NO _x to nitrogen N ₂ and oxygen O ₂ . In stoichiometric or slightly lean operation, unburned hydrocarbons HC and carbon monoxide CO are oxidized to carbon dioxide CO ₂ and water H ₂ O. At a substantially stoichiometric exhaust gas atmosphere, that is, at a λ of 1 or near 1, these conversions run almost completely. Such catalytic coatings are known and customary in the prior art from the exhaust aftertreatment of gasoline engines. Structure and operation of 3-way catalysts are thus well known in the art and require no further explanation.

The exhaust duct 16 may contain various sensors, in particular gas and temperature sensors. Shown here is a lambda probe 26, which is arranged at a position close to the engine in the exhaust gas channel 16. The lambda probe 26 can be configured as a step response lambda probe or as a broadband lambda probe and, in a known manner, enables the lambda control of the internal combustion engine 10, for which purpose it measures the oxygen content of the exhaust gas.

The signals detected by the various sensors, in particular the exhaust lambda measured with the lambda probe 26, enter an engine control unit 28. Similarly, various parameters of the internal combustion engine 10, in particular the engine speed and the engine load are read from the engine control unit 28. Depending on the various signals, a controller implemented in the engine control unit 28 thus controls the operation of the internal combustion engine 10, in particular regulating the fuel supply and the air supply such that a desired fuel mass and a desired air mass supplied to represent a desired air-fuel mixture (the exhaust target lambda). The air-fuel mixture is determined as a function of the operating point of the internal combustion engine 10, in particular the engine speed and the engine load from maps.

To improve the cleaning effect of the 3-way catalytic converter 20, it is provided that the internal combustion engine 10 is operated continuously with a substantially stoichiometric average lambda value, wherein the air-fuel ratio supplied to the internal combustion engine 10 with a predetermined oscillation frequency and a predetermined oscillation amplitude around this mean lambda value is periodically alternately deflected in the direction of a lean lambda value and a lambent lambda value (so-called lambda modulation). The oscillation frequency and the oscillation amplitude are selected so that the 3-way catalyst 20 is regenerated quasi-continuously.

In the present case, a continuous stoichiometric operation of internal combustion engine 10 is understood to mean that it is not switched back and forth between a standard operating mode and a regeneration operating mode, as is conventional in the prior art, but is operated virtually over its entire operating range in the illustrated stoichiometric mode with the lambda oscillation , Preferably, the internal combustion engine is driven over at least 98% of all stored in the operating map of the controller 28 operating points in the illustrated stoichiometric operation and this is not interrupted by regeneration intervals.

Furthermore, the term quasi-continuous regeneration of the 3-way catalytic converter 20 is understood to mean that its load state remains substantially constant and in particular at an extremely low level. This means that in the time average during a time interval in the size range less lambda oscillations no increasing loading of the 3-way catalyst 20 takes place. Preferably, a limit of at most 50% of the maximum load of the 3-way catalyst 20 is not exceeded.

The oscillation frequency and the oscillation amplitude are further selected so that a minimum conversion rate of unburned hydrocarbons (HC) and / or carbon monoxide (CO) and / or nitrogen oxides (NO _x ) is present at the 3-way catalytic coating 22, wherein the minimum conversion rate of statutory Limit values.

In most cases, the oscillation frequency is determined as a function of a current operating point of the internal combustion engine 10, in particular as a function of the engine load and / or engine rotational speed. The oscillation amplitude can also be determined as a function of the OSC.

In response to the various signals accumulating on the engine control unit 28, a controller implemented in the engine control unit 28 thus controls the operation of the internal combustion engine 10 to represent a desired exhaust target lambda.

Controllers automatically influence one or more physical variables to a predetermined level while reducing disturbing influences. For this purpose, controllers within a control loop continuously compare the signal of the setpoint with the measured and returned actual value of the controlled variable and determine from the difference of the two variables - the control deviation (control deviation) - a manipulated variable which influences the controlled system in such a way that the control deviation becomes a minimum , Because the individual control circuit elements have a time response, the controller must increase the value of the control deviation and at the same time compensate for the time behavior of the path so that the controlled variable reaches the desired value in the desired manner. Incorrectly set controllers make the control loop too slow, lead to a large control deviation or to undamped oscillations of the controlled variable and thus possibly to the destruction of the controlled system. In general, the controllers are distinguished according to continuous and unsteady behavior. Among the best-known continuous controllers are the "standard controllers" with P, PI, PD and PID behavior.

For the purposes of the present invention, a linear proportional, integral and derivative (PID) controller is preferably used. The PID controller therefore consists of the proportions of the P-element, the I-element and the D-element. The P element provides a contribution to the manipulated variable, which is proportional to the control deviation. The I-element acts by temporal integration of the control deviation on the manipulated variable with a weighting by the reset time. The D-element is a differentiator that is only used in conjunction with regulators with P and / or I behavior as a controller. He does not react to the level of the control deviation, but only to the rate of change.

According to the lambda modulation is carried out as in FIG. 6 represented (lambda actual value from the probe signal: rich dark curve, control variable of the controller: rich light curve, lambda reference ranges: bright rectangles).

At time t1, the switching of the controller direction takes place. First, there is a pre-controlled P-jump (P-component to reach the setpoint). The size of the P-jump can depend on various parameters. Among others, the P-jump may be dependent on a fixed desired amplitude. In a preferred embodiment, it may be determined here which portion of the setpoint desired amplitude is to be displayed via the P-jump. In addition, the current distance of the probe signal or a quantity derived therefrom (preferably lambda) can be evaluated by the current or future target value or target range, and the P jump can additionally be made dependent on this distance. In a particularly preferred embodiment, therefore, the magnitude of the P-jump is determined, which is necessary in order to arrive at the future setpoint value from the current lambda actual value, the desired setpoint containing the predetermined proportion, which of the setpoint desired amplitude is assigned to the P-set. Jump was assigned.

Between times t1 and t2, the controller is further adjusted with a fixed I component. From stored data, the runtime and the probe response time is known. Therefore, the I-component is determined so that at time t2 (without further interference) the probe signal or a quantity derived therefrom (preferably lambda) is expected to reach the target value or the target range, which means setting the full desired nominal amplitude , Thus, the I component depends on both the track characteristics, as well as the fixed portion of the amplitude to the P-jump, since the difference between the total amplitude and the fixed portion of the amplitude for the P-jump over the I component must be adjusted until the time t2.

From the time t2 is now switched from the pilot-controlled regulator setting to a (continuous) control, which is based on the difference between the actual value and the desired value of the probe signal or a derived quantity (preferably lambda).

Thus, the method combines the advantages of feedforward control and (continuous) control. For example, the data stored to characterize the track behavior may behave as in FIG. 4 consider at time t4. Overshoots are therefore avoided, and both lambda and controller value remain stable. At the same time a rapid control speed and a defined oxygen input or discharge is maintained in the catalyst, since after the distance reaction times switched to a fast controller whose parameters can be set independently of any inertia in lambda = 1 point of the probe.

Furthermore, with the method according to the invention, the dynamics of the probe can also be determined very simply and with good accuracy. Since the controller switching takes place in a controlled manner via a P-jump and an I-component and during the time of this pre-controlled control, the probe signal is not evaluated for regulation, the in FIG. 7 Lambda actual value from the probe signal: rich dark curve, control variable of the controller: bold bright curve, engine lambda nominal value: narrow rectangular curve, Δt _s : step response time).

In a preferred embodiment, depending on the size of the P-jump or the mixture adjustment made up to the time of the step response time determination, a minimum reaction of the probe is defined in comparison to the state before the controller switchover. This can be, for example, a signal change which corresponds to 20 to 50%, preferably 30%, of the pilot-controlled mixture adjustment. The step response time is now the time that has passed since the controller jump until the minimum reaction of the probe has been reached.

In a preferred embodiment, the actual time of the controller switching is not used exactly as the time of the controller switching for the determination of the minimum reaction of the probe, but taking into account the known line parameters of the reference value of the probe is determined only at a definable later date, which is after the controller switching, but before the modified mixture reaches the probe. Thus, dynamic mixture variations, which may have occurred immediately before the controller changeover in the engine, are taken into account and do not lead to a falsification of the step response times. In a further preferred refinement, a valid step response time is only determined if the pilot-controlled regulator adjustment had at least one predeterminable minimum size.

In a further preferred embodiment, after a predeterminable minimum time has elapsed since the controller switchover, without the probe showing the specified minimum reaction, the current time or a substitute variable is also evaluated as a valid step response time. Thus, the case is taken into account that the probe signal has a consistently constant value due to an error, that is, the minimum response would never be reached and thus no step response time would be determined.

From the determined step response time, the stored distance dead time can be deducted and so the pure probe reaction time can be determined. The probe response time may be used to generate a maintenance signal if this or a quantity derived therefrom exceeds defined thresholds. The probe reaction time for evaluation can be considered separately after fat-lean jump and lean-fat jump.

A further advantage of the method according to the invention is that with dynamically deteriorating probes the in FIG. 4 Overshoot described at the times t1 and t2 can be easily avoided, so that the inventive method has a higher stability and robustness against dynamically deteriorating probes than previously known methods.

For dynamically only slightly worse probes, can be used to determine the time t2 in FIG. 6 , ie switching to the fast controller, a certain certainty can be added to the runtime parameters. This can be done, for example, by multiplicative and / or additive values. Switching to the fast controller is done a little later than would be possible with a fast sensor, but only if a slower reacting sensor had arrived at the signal target value.

In a further embodiment, the probe reaction time determined as described above can be used to adapt the control method. For this purpose, at least one, preferably the larger of the two probe reaction times (that is to say reaction times separated according to fat-lean or lean-fat jump) is used. From this probe reaction time, suitable timers for the route parameters are preferably derived. In this case, the determination of the time t2 in FIG. 6 , ie switching to the fast controller, taking into account the determined probe reaction time so that the probe signal or a derived quantity (preferably lambda) has reached the target value at this time.

In a further preferred refinement, the control parameters of the subsequently activated, continuous control are adapted to the probe reaction time. In particular, the controller can be made slower for a dynamically worse probe and thus overshoots can be avoided.

LIST OF REFERENCE NUMBERS

10

Internal combustion engine

16

exhaust duct

20

3-way catalyst

22

3-way catalytic coating

26

lambda probe

28

Engine control unit

Δt _s

Step response time

Claims (7)

1. Method for operating an internal combustion engine (10) in which exhaust gas produced by the internal combustion engine (10) is passed via a 3-way catalytic converter (20) disposed in the exhaust duct (16) and a lambda probe (26) detects a variable that is characteristic of an exhaust gas lambda number before the 3-way catalytic converter (20) and forwards the same to an engine controller (28) with integrated PI controller or PID controller, wherein an essentially stoichiometric exhaust gas lambda number is set up with the PI controller or PID controller of the engine controller (28) by specifying a target value and the exhaust gas lambda number is alternately deflected towards a weak lambda number and a rich lambda number with a specified periodic target value variation such that at the start of each target value variation a precontrolled P component with following I component is specified up to a point in time t2, wherein the point in time t2 is specified using stored parameters characterizing a path behavior so that at said point in time t2 the probe signal or a variable derived therefrom must have reached the specified target value, characterized in that from the point in time t2 a changeover to control based on a difference between an actual value and the target value of the lambda probe (26) or a variable derived therefrom takes place for a specifiable time period until the end of the respective target value variation.
2. Method according to Claim 1, characterized in that for determining a response time of the lambda probe (26) a minimum response of the lambda probe (26) is defined in comparison to the state before the controller changeover and the time that has passed between the controller changeover and the minimum response of the lambda probe (26) is recorded as the response time.
3. Method according to Claim 2, characterized in that the response time is only determined if the target value specified by the PI controller or PID controller exceeds a specified minimum magnitude.
4. Method according to Claim 2 or 3, characterized in that the response time of the lambda probe (26) is recorded separately for a rich-weak step and a weakrich step.
5. Method according to any one of the preceding claims, characterized in that a magnitude of the P component is specified depending on a target amplitude of the target value variation.
6. Method according to Claim 5, characterized in that the I component is specified such that the probe signal or a variable derived therefrom has reached the target value at the point in time t2.
7. Engine controller (20) for controlling an operation of an internal combustion engine (10), which is set up to perform the method according to any one of Claims 1 to 6.

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