DE102010000928B3 - Method for approximately adjusting air/fuel ratio in exhaust gas of e.g. diesel engine, involves determining fuel path reference lambda value, and adjusting air/fuel ratio under consideration of values for reaching lambda-set value - Google Patents

Abstract

The method involves determining a fuel path reference defining lambda value from a lambda-set value. Change of fuel quantity is determined such that an actual lambda value approximately reaches the defining lambda value. Change in air quantity is determined such that actual lambda value is obtained when the lambda-set value and the fuel path reference defining lambda value does not conform with each other under consideration of the determined change of the fuel quantity. Air/fuel ratio is adjusted under consideration of changed values for approximately reaching the lambda-set value. An independent claim is also included for a device for approximately adjusting air/fuel ratio in exhaust gas of an internal combustion engine to a preset lambda-set value.

Description

The present invention relates to a method for adjusting an air / fuel ratio in the exhaust gas of an internal combustion engine according to the preamble of claim 1.

Furthermore, the invention relates to a device for carrying out a corresponding method.

The present invention can be used both in conjunction with gasoline spark ignition engines and diesel engines. Furthermore, the engines may be followed by various exhaust gas treatment arrangements such as three-way catalytic converters, NO _x traps or diesel particulate filters. In the following, when referring to a lambda value, it is usually meant the relative air / fuel ratio that characterizes the deviation from the stoichiometric combustion (λ = 1).

The control of the air / fuel ratio in the exhaust gas of an internal combustion engine is a widely used measure, which serves to limit emissions (for example, limiting soot formation at full load) and, on the other hand, to maintain the performance of exhaust treatment systems, for example, a three-way catalytic converter an Otto engine with spark ignition or in connection with an enriched operation, which is required for the regeneration of a NO _x trap. Further, lambda control is also used to protect exhaust treatment components by preventing exothermic reactions, for example, to prevent unwanted soot combustion in a diesel particulate filter.

For these diverse control tasks, a desired lambda value is usually predetermined by a higher-level engine management system, which is then set by changing the fuel and / or air metering and adjusted by comparison with a measurement of the lambda value in the exhaust gas (usually by means of a lambda probe).

Since a pure feedback control has proved to be too sluggish and inaccurate given the existing delays on the controlled system - for example due to the exhaust paths and the sensor inertia and also in view of the difficult control conditions in the frequently occurring transient engine operating conditions, it is well known for this purpose Use controller with feed forward or feedforward term, in which the required for a desired lambda desired value change of the fuel and Luftzumessung initially calculated by an inverse model of the controlled system approximately, impressed and then only the remaining difference to the actual value -. B. by means of a PID controller - is corrected.

A desired air / fuel ratio can be adjusted both by changing the fuel and the Luftzumessung. A change in the fuel metering has a significantly faster effect on the air / fuel ratio in the exhaust than a change in the air metering, which is why a change in the fuel metering is often preferred, especially in transient engine conditions to compensate for effects caused by delays in the air path. Furthermore, an adjustment of the fuel metering is usually used to compensate for deviations of the fuel quantities in the injection system, which may differ from the calibrated standard values either due to manufacturing tolerances or due to wear.

On the other hand, with a setting of lambda by changing the fuel metering also disadvantages: For example, the quality of combustion must not be significantly affected; In addition, should be avoided by the driver perceivable torque surges, if the engine is just transmitting power. Furthermore, the exhaust gas temperatures - especially with regard to a turbocharger - should be limited. Finally, excessive oil dilution and excessive fuel consumption should be avoided.

Regardless, in the prior art lambda adjustment is primarily accomplished by adjusting fuel metering, particularly for rapid changes in the presence of transient engine operating conditions. Possibly. in case of larger deviations, the air metering will later be adapted; but on a larger time scale. This "subsequent" adaptation of the air metering also does not change the fact that, in the case of transient conditions, the problems described above associated with a change in fuel metering initially occur.

From the DE 10 2006 053 104 A1 a method for adapting a characteristic map in the context of an internal combustion engine control is known, in which for rapid achievement of a predetermined lambda target value in a transition from a lean to the rich operation in principle both a Luftmengen- and a fuel injection amount can be precontrolled, from this document However, it remains unclear how an optimally coordinated control of these two manipulated variables should be achieved quantitatively.

The present invention is therefore based on the object to provide a method and an apparatus for adjusting or regulating the air / fuel ratio in the exhaust gas of an internal combustion engine, which works with an optimally coordinated change of the fuel and Luftzumessung, so that the above be avoided as possible problems described. This is to achieve an optimal and steady transition between influencing fuel metering (for rather minor changes) and influencing air metering (for larger changes in air / fuel ratio).

The solution of the above object is achieved according to the features of the independent claims 1 and 6, respectively.

Advantageous embodiments of the present invention are described in the subclaims.

As part of a method according to the invention, the following steps are provided, wherein the order of the steps - as far as possible - can also be modified:
Starting from a predetermined lambda setpoint value λ _des , a so-called "fuel path-related lambda interval" is first determined with a lower limit λ _{narrow min} and an upper limit λ _{narrow max} . This interval characterizes the Lambda range, which can be achieved only by changing the fuel metering or - to avoid the aforementioned problems - should.

The determination of the limits of this lambda interval is preferably based on prestored table memory or predetermined functional relationships depending on the current engine operating conditions, in particular depending on the current engine speed and the load (or the torque) and preferably also depending on the engine temperature. Furthermore, the operating mode of the internal combustion engine is preferably taken into account (eg lean operation, stratified charge operation (if present), regeneration operation for a nitrogen oxide trap, etc.), which depends on scaling, offset values compared to the basic values stored in the tables or else on a selection of different tables can be realized by the operating mode. The lambda interval is preferably not (or at least not directly) dependent on the current lambda value or on the preset lambda desired value.

In the context of a preferred embodiment of the invention, a further lambda interval, which is referred to as "generally limited lambda interval" and which is wider than the fuel path-related limited lambda interval, is determined on the basis of predetermined engine operating parameters, wherein the fuel path-related limited lambda interval is contained in this broader interval , The predetermined desired lambda value is preferably limited to values from this interval, ie if the desired value is greater or less than the respective interval limit λ _{wide max} or λ _{wide min} , this is equated with the corresponding interval limit, this limited value then being used for the further steps used as setpoint. With this limitation, the absolute lambda setting interval, whether set by changing the fuel or air supply, is limited to a reasonable range.

The determination of this broader generally limited interval is preferably analogous to the determination of the fuel path related limited interval, i. H. based on various engine parameters.

The dimensioning of these two prescribed intervals takes place taking into account the criteria mentioned above. Thus, the fuel path-related limited interval, for example, each selected so narrow that perceptible for the driver torque shocks are avoided.

In the context of the inventive process, the next step is a determination of a fuel path relative limited lambda target value λ _{of the lim K} from the predetermined lambda target value λ _of by the predetermined lambda desired value upwards to the upper limit _{narrow max} of the fuel path related limited Lambda interval is limited λ and / or predetermined lambda setpoint is limited down to the lower limit λ _{narrow min of} the fuel path related limited lambda interval. In particular, a limitation of the lambda desired value upward to the upper limit λ _{narrow max can} preferably take place when the lambda desired value is greater than or equal to a measured lambda value λ _meas . A limitation of the desired lambda value downwards to the lower limit λ _{narrow min} may preferably take place when the lambda desired value is smaller than the measured lambda value λ _mess .

Subsequently, a determination of the required change in the amount of fuel Δm _{K is made} such that the actual lambda value approximately reaches the lambda desired value λ of _the welcome _K that is limited by the fuel path.

If the original target value has been already within the fuel path relative limited Lambda interval λ _of are equal λ _of (or λ _{of the} lim) and λ _{the lim K} and lambda adjustment is made only with reference to the metering of fuel and the air quantity remains unchanged.

On the other hand, if the lambda setpoint value λ of _the λ (or λ of _{the μ} , considering the generally limited interval) and the lambda setpoint lambda setpoint λ of _{the begr K} , which is limited by the fuel _path , do not match (ie λ of _the limit _K has been limited), the determination of the required change takes place as a further step the amount of air Δm _L such that the actual lambda value, taking into account the change in fuel quantity Δm _K determined according to the above step, reaches the lambda setpoint value λ _of or λ of _the limit.

Finally, adjustment of the air and fuel values taking into account the above-defined change values Δm _K and possibly Δm _L for (at least approximately) reaching the lambda setpoint value λ _of or λ of _{the threshold is carried out} .

The method according to the invention is particularly preferably used in the context of a control with feed forward or feedforward term. For this purpose, by means of the method described above, the respectively required fuel and possibly air change is calculated on the basis of an inverse model of the air / fuel system and adjusted within the scope of a feed forward or feedforward term. The possibly remaining difference between the current lambda value λ _mess and the lambda setpoint λ _des is regulated by a control element, for example via a PID controller, whereby this PID controller preferably primarily controls the fuel metering.

Furthermore, a device designed for carrying out the method is proposed in the context of the invention.

The device is designed in terms of hardware as known in the art and has u. a. Actuators for influencing the fuel metering (eg electronically controllable injection pump) as well as for influencing the air metering (eg electronic throttle valve). The current lambda value is measured by means of a corresponding lambda probe in the exhaust gas.

The implementation of the method according to the invention is preferably carried out within a microprocessor-controlled engine control unit, which receives as input the desired lambda setpoint from a higher-level control entity. The method described above is accordingly preferably implemented by means of program code and by means of correspondingly prestored tables.

• i) Zunächst wird λ_des als Sollwert vorgegeben und der aktuelle Lambdawert λ_mess gemessen.
• ii) abhängig insbesondere vom Drehmoment und der Drehzahl des Motors sowie abhängig vom Betriebsmodus werden anhand von Tabellenspeichern zwei Lambda-Bereiche festgelegt, nämlich ein breiter, allgemein begrenzter Bereich mit oberer und unterer Grenze λ_{breit min} und λ_{breit max}, sowie ein kraftstoffpfadbezogen begrenzter, schmalerer Bereich mit den Grenzen λ_{schmal min} und λ_{schmal max}. Der kraftstoffpfadbezogen begrenzte Bereich stellt den λ-Bereich dar, in dem eine Anpassung auf den Sollwert (ausschließlich) mittels des Kraftstoffpfades erfolgen soll.
• iii) Aus λ_des wird zunächst ein auf den allgemein begrenzten Bereich begrenzter Sollwert λ_{des begr} bestimmt, gemäß λ_{des begr} = max(min(λ_des, λ_{breit max}), λ_{breit min}) (1) Diese Begrenzung stellt sicher, dass im Rahmen der späteren Feedforward-Korrektur nur eine begrenzte Änderung der Luft/Kraftstoffzumessung erfolgen kann.
• iv) Ferner wird ein noch weiter begrenzter Sollwert λ_{des begr K} bestimmt, der die maximal mögliche Anpassung über den Kraftstoffpfad charakterisiert, und zwar wie folgt: – wenn der unter iii) bestimmte Wert λ_{des begr} größer/gleich dem gemessenen λ-Wert λ_mess ist, wird λ_{des begr K} gleich λ_{des begr} gesetzt, jedoch nach oben hin auf die obere Grenze λ_{schmal max} des schmalen Bereichs hin begrenzt, d. h. λ_{des begr K} = min(λ_{des begr}, λ_{schmal max}) (2) oder, – wenn der gemäß Schritt iii) bestimmte Wert λ_{des begr} kleiner als der gemessene λ-Wert λ_mess ist, wird λ_{des begr K} gleich λ_{des begr} gesetzt, jedoch nach unten hin auf die untere Grenze λ_{schmal min} des schmalen Bereichs hin begrenzt, d. h. λ_{des begr K} = max(λ_{des begr}, λ_{schmal min}) (3).
• v) Nunmehr wird die notwendige Kraftstoffkompensation berechnet, um den gemessenen λ-Wert auf den vorstehend bestimmten, kraftstoffmäßig begrenzten Sollwert λ_{des begr K} einzustellen, gemäß

mit

Δm_K

Kraftstoffmassenstrom-Anpassung;

m_L

Lustmassenstrom;

λ_Precal

vorkalibriertem Lambdawert;

Der vorkalibrierte Lambdawert λ_Precal wird aus den nicht kompensierten Feedforward-Adaptionen für den Luft- und den Kraftstoffpfad berechnet.

• vi) Anschließend wird die verbleibend erforderliche Korrektur des Luftmassenstroms Δm_L wie folgt berechnet: Δm_L = (λ_{des begr K} – λ_Precal)·m_{K Precal}·14.7 + λ_{des begr}·Δm_K·14.7 (5), wobei m_{K Precal} der λ_Precal zugrundeliegende Kraftstoffmassenstrom ist.
• vii) Schließlich werden die so näherungsweise mittels Feedforward-Korrekturen vorgesteuerten Luft- und Kraftstoffwerte – beispielsweise mittels eines PID-Regelgliedes – noch anhand des Meßwertes λ_mess auf den Sollwert λ_des geregelt.

An embodiment of a method according to the invention will be explained in more detail below:

• i) First, specified as a target value _of λ and the current lambda value λ _measured measured.
• ii) Depending on the torque and the speed of the engine as well as on the operating mode, two lambda ranges are defined on the basis of table memories, namely a broad, generally limited range with upper and lower limits λ _{broad min} and λ _{wide max} , and a fuel path related limited, narrower range with the limits λ _{narrow min} and λ _{narrow max} . The fuel path-related limited range represents the λ range in which an adaptation to the desired value (exclusively) by means of the fuel path is to take place.
• iii) For _the λ is determined first, a limited on the generally limited range _{of the} target value λ _lim, according λ of _{the bur} = max (min (λ _des , λ _{wide max} ), λ _{wide min} ) (1) This limitation ensures that only a limited change in the air / fuel metering can take place during the later feedforward correction.
• iv) Furthermore, a still further limited setpoint λ of _{the limit K is} determined, which characterizes the maximum possible adaptation via the fuel path, namely as follows: if the value λ of _the value determined in iii) is greater than / equal to the measured λ value λ is _measured, is set equal to _K λ _{lim of the} λ _lim, but upward to the upper limit λ _{max narrow} the narrow region limited way, that λ of _{the log K} = min (λ of _{the bur} , λ _{narrow max} ) (2) or, - when is the value determined λ _{of the lim} smaller than the measured λ-value λ _measured according to step iii), is set to λ _{of the lim K} equals λ _{of the lim,} but downward to the lower limit λ _{narrow min} of the narrow region limited, ie λ of _{the limit K} = max (λ of _{the bur} , λ _{narrow min} ) (3).
• v) Now, the necessary fuel compensation is calculated to set the measured λ value to the above-determined, fuel-limited setpoint λ of _{the limit K} , in accordance with FIG

With

Δm _K

Fuel mass flow adjustment;

m _L

Lust mass flow;

λ _Precal

pre-calibrated lambda value;

The pre-calibrated lambda value λ _Precal is calculated from the _{uncompensated} feedforward adaptations for the air and fuel paths.

• vi) Subsequently, the remaining required correction of the air mass flow Δm _L is calculated as follows: Δm _L = (λ of _{the func K} - λ _Precal ) · m _{K Precal} · 14.7 + λ of _the fundamental _K · 14.7 (5), where m _{K Precal is} the fuel _{mass flow} underlying λ _Precal .
• vii) Finally, the air and fuel values precontrolled by means of feedforward corrections-for example, by means of a PID control element-are still regulated to the desired value λ _of the measured value λ _mess .

In one example, in a combustion engine with NO _x trap for a regeneration of this nitrogen oxide trap, a lambda value of z. B. 0.97 (enriched) required. The fuel path-related limited interval in this case could be between 1.08 and 1.10. Now, if the current lambda value is 1.09, then the change is made from 1.09 to 1.08 over the fuel path and the change from 1.08 to 0.97 over the air path.

Claims (6)

A method for at least approximately setting an air / fuel ratio in the exhaust gas of an internal combustion engine to a predetermined lambda desired value λ _des , characterized by the following steps: a) determination of a lower and an upper limit λ _{narrow min} , λ _{narrow max} for a fuel path lambda interval based on predetermined engine operating parameters; b) determining a fuel path relative limited lambda target value λ _{of the lim K} from the predetermined lambda target value λ _of by the predetermined lambda desired value upwards to the upper limit _{narrow max} of the fuel path related limited Lambda interval is λ limited and / or the predetermined lambda setpoint downwardly on limiting the lower limit λ _{narrow min of} the fuel path related limited lambda interval; c) determining a change in the amount of fuel Δm _{K in} such a way that the actual lambda value at least approximately reaches the lambda desired value λ of _the begr _K bounded by the fuel path; d) if the lambda nominal value do not match λ _of and the fuel path relative limited lambda target value λ _{of the lim K,} determining a change in the air amount Dm _L such that the actual lambda value reaches the lambda target value λ _of taking into account the determined according to step c) change of the fuel amount, and e) adjusting the air and fuel values taking into account the change values Δm _K and possibly Δm _L for approximately reaching the lambda setpoint λ of _the . A method according to claim 1, characterized in that based on predetermined engine operating parameters, a lower and upper limit λ _{width min} , λ _{width max is determined} for a generally limited lambda interval, wherein the generally limited lambda interval is wider than the fuel path related limited lambda interval and the fuel path related limited lambda interval includes, and that for the process steps a) to e) instead of the predetermined lambda desired value λ _of a limited lambda desired value λ of _{the bur} is used, which is limited to lambda values from the generally limited lambda interval. A method according to claim 1 or 2, characterized in that the fuel path related limited and / or the generally limited lambda interval based vorgespeicherter tables and / or vorgespeicherter functional relationships depending on at least one of the parameters from the group of engine speed, engine load or engine torque, an engine operating mode , such as the combustion mode or a regeneration mode for an exhaust treatment device, and / or the engine temperature is determined. Method according to one of claims 1 to 3, characterized in that a current lambda value λ _{mess is} measured, and that the air / fuel ratio is controlled based on the current lambda value λ _mess with a pilot or feed forward term to the lambda setpoint λ _of , wherein in the process steps c) and d) the respectively required fuel and air change is calculated on the basis of an inverse model of the air / fuel system and adjusted within the pilot control or feedforward term, and that a remaining difference between the current lambda value λ _mess and the lambda setpoint λ of _the regulated via a control element. A method according to claim 4, characterized in that in step b) of the method, a limitation of the lambda setpoint up to the upper limit λ _{narrow max of} the fuel path Lambda interval is then when the lambda setpoint is greater than or equal to the measured lambda value λ _mess , and that a limitation of the lambda desired value down to the lower limit λ _{narrow min of} the fuel path-related lambda interval then takes place when the lambda desired value is smaller than the measured lambda value λmess. Device for adjusting and / or regulating an air / fuel ratio in the exhaust gas of an internal combustion engine, characterized in that the device is designed for carrying out a method according to one of claims 1 to 5.