[0001] The invention relates to a method and a control system for an internal combustion
engine with a three-way catalyst.
[0002] In order to comply with present and future emission legislation for automotive vehicles,
it is extremely important to operate exhaust gas aftertreatment devices with optimal
efficiency. Thus it is well known for three-way catalysts (TWC; they remove simultaneously
emissions of hydrocarbons HC, nitrogen-oxides NOx and carbon monoxide CO from the
exhaust gas) to control the air-fuel-ratio or lambda-value behind the catalyst in
a feedback loop. Due to delays in said control loop it is however possible that the
catalyst is not working optimally during transient conditions in the exhaust gas.
[0003] The US-patent application
US 2004/0040286 A1 (application date August 30, 2002, publication date March 4, 2004) discloses a system
and a method for controlling an engine to regulate the oxygen storage level in an
emission control device. The system includes oxygen sensors disposed in an exhaust
gas stream of the engine upstream and downstream of the emission control device. The
oxygen sensors generate a feedgas air fuel signal and a tailpipe air fuel signal.
The system further includes an electronic control unit configured to obtain an adjusted
feedgas air fuel ratio responsive to the feedgas air fuel signal and the tailpipe
air fuel signal in order to correct any bias in the feedgas air fuel signal. The electronic
control unit is further configured to obtain an estimate of an oxygen storage level
in the emission control device responsive to the adjusted feedgas air fuel ratio and
the tailpipe air fuel signal. Finally, the electronic control unit is configured to
generate a control signal for the engine responsive to the adjusted feedgas air fuel
ratio and the oxygen storage level estimate for the emission control device.
[0004] Based on the above mentioned situation it was an object of the present invention
to provide means for an improved and inexpensive control of an internal combustion
engine with a three-way catalyst that are particularly able to deal with transient
conditions.
[0005] This object is achieved by a control system according to claim 1 and a method according
to claim 7. Preferred embodiments are disclosed in the dependent claims.
[0006] According to its first aspect the invention relates to a control system for an internal
combustion engine with a three-way catalyst (TWC). Said control system may be implemented
by means known in the art, for example by a microprocessor with associated software
or by dedicated electronic circuits. The control system comprises the following components
in a parallel (i.e. not cascaded) arrangement:
- a) An oxygen-storage-controller that is adapted to control the internal combustion
engine such that the level of oxygen stored in the TWC lies within predetermined oxygen-storage-limits.
The level of stored oxygen may particularly be expressed as a fraction or a percentage
of the maximal amount of oxygen that can be stored in the TWC. Typically the oxygen
storage level in the TWC is desired to lie between 10% to 60%, preferably 20% to 50%.
- b) A lambda-controller that is adapted to control the internal combustion engine such
that the lambda value of the exhaust gas within or behind the TWC lies within predetermined
lambda-limits. As usual, the lambda value (λ) is defined as the amount of air present
in a gas relative to the amount of air that is needed for the combustion of the fuel
present in the gas. Thus values λ > 1 correspond to a lean, values λ < 1 to a rich,
and the value λ = 1 to a stoichiometric air-fuel mixture. It is known in the state
of the art that the lambda-limits for an optimal efficiency of a TWC are close to
the stoichiometric value, for example with λlow = 0.98 and λhigh = 1.02.
Both the oxygen-storage-controller and the lambda-controller may for example steer
the internal combustion engine by determining the (desired) air-fuel-ratio of the
combustion in the engine.
[0007] The control system described above takes the level of oxygen stored in the TWC into
account and guarantees that it lies within predetermined optimal limits. These limits
can be determined such that the TWC behaves robust with respect to transient deviations
of the exhaust gas composition from the optimal value, i.e. such that the TWC does
not become severely ineffective if the exhaust gas is momentarily too rich or too
lean.
[0008] As the oxygen-storage-controller and the lambda-controller pursue separate objectives,
their control activities must be coordinated,hence it is preferred to give the oxygen-storage-controller
priority. Thus the lambda-controller is only operative if the level of oxygen in the
TWC lies within the predetermined oxygen-storage-limits such that the oxygen-storage-controller
is idle.
[0009] The lambda-controller preferably operates in a closed loop comparing the measured
lambda value within or behind the TWC with a desired lambda value. For the measurement
of the lambda value, the control system optionally comprises at least one heated exhaust
gas oxygen (HEGO) sensor as it is well known in the state of the art.
[0010] In a preferred embodiment of the invention, the oxygen-storage-controller is linked
to a catalyst-model that is used to predict the level of oxygen stored in the TWC.
Thus there is no need for additional sensors or other expensive equipment in order
to measure the oxygen content directly.
[0011] The aforementioned catalyst-model preferably receives as input signals the lambda
value, the mass flow and/or the temperature of the exhaust gas in front of and/or
behind the TWC. Based on these variables, the catalyst-model can estimate the level
of oxygen stored in the TWC with good precision.
[0012] In order to improve the reliability of the catalyst-model, it may comprise an adaptation
unit that is able to adjust the catalyst-model based on a comparison between the modeled
and the measured lambda value within or behind the TWC. Said lambda value can be readily
derived from the catalyst-model additionally to the required prediction of the level
of stored oxygen. The measured lambda value is normally already available, too, as
the feedback signal for the lambda-controller.
[0013] The control system may furthermore comprise a memory (e.g. ROM, RAM, hard disc etc.)
in which the predetermined oxygen-storage-limits and/or the lambda-limits are stored
as a function of engine operating parameters. Thus the control system can always use
the optimal parameters for the prevailing conditions, wherein said limits are preferably
determined during a calibration procedure. The engine operating parameters on which
the limits depend may particularly comprise the mass flow and the temperature of the
exhaust gas entering the TWC.
[0014] The invention further relates to a method for the control of an internal combustion
engine with a three-way catalyst (TWC) which comprises the following steps, which
are executed in parallel:
- a) Controlling the internal combustion engine such that the level of oxygen stored
in the TWC lies within predetermined oxygen-storage-limits.
- b) Controlling the internal combustion engine such that the lambda value of the exhaust
gas within or behind the TWC lies within predetermined lambda-limits.
[0015] To avoid conflicts, a lambda-controller for controlling the lambda-value (λ
c) is only operative if the level of oxygen stored in the TWC lies within the predetermined
oxygen-storage-limits (ζ
low, ζ
high).
[0016] The method comprises in general form the steps that can be executed with a control
system of the kind described above. Therefore, reference is made to the preceding
description for more information on the details, advantages and improvements of that
method.
[0017] According to a preferred embodiment of the method, the level of oxygen stored in
the TWC is modeled as a function of engine operating parameters, for example of the
mass flow, the temperature and/or the lambda value of the exhaust gas entering and/or
leaving the TWC.
[0018] The aforementioned modeling may be further improved if it is adapted based on a comparison
between a modeled value and the corresponding measured value of an operating parameter,
wherein said operating parameter may particularly be the lambda value of the exhaust
gas leaving the TWC.
[0019] In the following the invention is described by way of example with reference to the
accompanying Figures, in which
- Fig. 1
- shows a schematic representation of a control system according to the present invention;
- Fig. 2
- shows the conversion efficiency of a TWC with respect to CO, HC, and NOx as a function
of the oxygen storage level ζ in the TWC.
[0020] Future emissions legislation is driving the complexity and cost of automotive vehicles
up. The main stay of achieving compliance currently resides with TWC technology in
the aftertreatment system, a topic that is addressed with the present invention.
[0021] Figure 1 schematically depicts an internal combustion engine 2 which produces exhaust
gas with a lambda value (i.e. normalized air-fuel-ratio) λ
e. The exhaust gas passes through a three-way catalyst TWC 4 in which the emissions
of carbon monoxide CO, hydrocarbons HC, and nitrogen oxides NOx are treated. In the
case shown in Figure 1, the TWC 4 consists of two bricks 4a, 4b, and the lambda value
λ
c of the exhaust gas within the TWC is measured between these two bricks by means of
a HEGO sensor 5. This sensor layout is typical of current systems. A layout with the
HEGO placed downstream of the entire catalyst volume would however possibly lead to
further optimized control in this instance.
[0022] The control system comprises a first control loop with a lambda-controller 7 that
shall guarantee operation of the TWC 4 with optimal efficiency under steady state
conditions. The lambda-controller 7 receives as input the difference between a desired
HEGO voltage, HEGO
ref, and the corresponding measured HEGO voltage, HEGO
mes, sensed by the HEGO sensor 5. The lambda-controller 7 then controls the internal
combustion engine 2 such that the difference (HEGO
ref-HEGO
mes) is minimized. This approach is based on the fact that optimum steady state conversion
efficiency from a TWC can be directly mapped against post/mid converter HEGO voltage
HEGO
mes (which is a function of catalyst lambda λ
c). Best conversion of all three pollutants CO, HC, and NOx is normally found between
HEGO
mes = 0.5-0.65 V. The converter operates then very close to true stoichiometry. The best
conversion lambda can be targeted under these conditions because the converter inlet
lambda can usually be guaranteed within tight limits and large lambda excursions need
not be considered. However, it is when controlling to post/mid converter HEGO reference
only that optimum buffering of input lambda excursions cannot be guaranteed as this
signal contains no information as to the amount of oxygen stored on the catalyst.
Moreover, the lambda-controller 7 always lags in response since it is purely a feedback
system and thus control action lags any observed error.
[0023] The control system of the present invention therefore further comprises a second
control loop, wherein a catalyst-model 6 estimates the oxygen storage level ζ
est within the TWC 4 based on inputs from the internal combustion engine 2. Said inputs
may for example comprise the mass flow m
F and the temperature T of the exhaust gas entering the TWC as well as the lambda value
λ
e at the entrance of the TWC that is measured by an UEGO sensor 3. Suitable realizations
of the model 6 may be found in literature (e.g.
Balenovic, de Bie, Backx: "Development of a Model-Based Controller for a Three-Way
Catalytic Converter", SAE paper no. 2002-01-0475, which is incorporated into the present application by reference). The oxygen storage
level ζ
est estimated by the catalyst-model 6 is compared to a reference storage level ζ
ref, and the difference (ζ
ref-ζ
est) between these values is fed to an oxygen-storage-controller 1. This oxygen-storage-controller
1 then determines the desired air-fuel-ratio λ
e_ref at the inlet of the internal combustion engine 2 in such a way that the oxygen storage
level ζ within the TWC lies within predetermined oxygen-storage-limits, i.e. ζ ∈ [ζ
low, ζ
high]. In order to guarantee a unique input to the internal combustion engine 2, a switch
8 is provided by which either the oxygen-storage-controller 1 or the lambda-controller
7 is connected to the internal combustion engine 2.
[0024] For checking the validity of the model 6, the downstream lambda signal predicted
by the model 6 (scaled with the HEGO sensor characteristic) is compared with the reading
HEGO
mes of the HEGO sensor 5 to estimate the model error Δ at a time instant. This model
error Δ is fed into an observer (i.e. Kalman filter with gain K) which updates the
predicted oxygen level in order to cope with noise, system biases and model uncertainties.
[0025] The control system described above uses an embedded model 6 to continuously drive
the level ζ of oxygen stored in the TWC to its optimal value, therefore guaranteeing
maximum robustness to air/fuel excursions. Once close to the set point, additional
control based on HEGO sensor signals via the lambda-controller 7 will provide optimum
catalyst conversion efficiency. Therefore, the original system performance is retained
while the robustness is improved.
[0026] The oxygen storage level ζ should typically approach 50% full (to buffer excursions
during transients) and when this criterion is satisfied the inlet lambda should be
controlled to that which results in the highest steady state conversion. Thus the
oxygen-storage-controller 1 is used in the first instance to maintain the oxygen store
between set limits ζ
low, ζ
high (to maintain high conversion during lambda excursions), and when within these limits
HEGO controller 7 will be used to further raise the conversion efficiency to the best
possible under steady state conditions by supplying the best input lambda reference.
As the oxygen storage estimate ζ
est is calculated on-line, the oxygen-storage-controller 1 can revert back to controlling
the oxygen storage at times when the store violates the set limits. The model prediction
of oxygen store is then used as feedback signal, and the error between the estimated
and reference oxygen store signal is fed to the controller 1 that drives the system
towards the desired oxygen store level.
[0027] During calibration an optimal steady state catalyst lambda is determined and related
to the corresponding HEGO voltage. It is selected on the basis of best three-way conversion
in the presence of little or no input excursions.
[0028] The actual reference steady state level of stored oxygen and set limits ζ
low, ζ
high can be determined on the basis of model conversion characteristics. Set limits determine
oxygen store levels ζ at which either HC/CO conversion during rich lambda excursions
or NOx conversion during lean inlet lambda excursions substantially decrease. Figure
2 shows the dependence of the conversion efficiency (vertical axis) for CO, HC, and
NOx in dependence on oxygen storage level ζ. In this example, a level between ζ
low = 10% and ζ
high = 40% demonstrates the best robustness at absorbing lean and rich excursions in input
lambda. This value is important as during normal operation the input lambda to the
catalyst can be subject to large excursions depending on driver demand. Optimal steady
state points can be stored as a map (function of exhaust flow and temperature) in
the control system or engine control unit (ECU).
[0029] The operation of the control system is divided into two modes: tracking and regulating
mode. The model 6 operates continuously in either of the two modes. As soon as the
inferred oxygen level ζ
est within the catalyst ceria leaves the interval [ζ
low, ζ
high] previously determined (as a consequence of various disturbances), the tracking-
or oxygen-storage-controller 1 is switched on. This mode uses the estimated oxygen
store level ζ
est as the feedback signal. The controller 1 sets the required engine air-fuel ratio
λ
e_ref, which is achieved by a standard air-fuel ratio engine controller placed in the inner
control loop, to return within the desired limits. Once the level of stored oxygen
returns to the predetermined interval [ζ
low, ζ
high] of the steady state level, the system switches to the regulating mode, which uses
the lambda-controller 7 with a direct feedback from the HEGO sensor. In this way the
controlled system achieves extra robustness and low susceptibility to small drifts
that are typical for such an application.
1. Control system for an internal combustion engine (2) with a three-way-catalyst TWC
(4), comprising a parallel arrangement of:
a) an oxygen-storage-controller (1) which is adapted to control the internal combustion
engine (2) such that the level of oxygen stored in the TWC (4) lies within predetermined
oxygen-storage-limits (ζlow, ζhigh);
b) a lambda-controller (7) that is adapted to control the internal combustion engine
(2) such that the lambda-value (λc) of the exhaust gas within or a behind the TWC (4) lies within predetermined lambda-limits,
characterized in that the lambda-controller (7) is only operative if the level of oxygen stored in the
TWC (4) lies within the predetermined oxygen-storage-limits (ζ
low, ζ
high).
2. The control system of claim 1, characterized in that it comprises a HEGO sensor (5) for measuring the lambda-value (λc) of the exhaust gas within or behind the TWC (4).
3. The control system according to one of claims 1 or 2, characterized in that it comprises a catalyst-model (6) to predict the level (ζest) of oxygen stored in the TWC (4).
4. The control system according to claim 3, characterized in that the catalyst-model (6) receives as input signal the lambda-value (λe), the mass flow (mF) and/or the temperature (T) of the exhaust gas entering and/or leaving the TWC (4).
5. The control system of claim 3 or 4, characterized in that the catalyst-model (6) comprises an adaptation unit that is able to adjust the model
based on a comparison between the modelled and the measured lambda-value (λc) within or behind the TWC (4).
6. The control system according to one of claims 1 to 5, characterized in that it comprises a memory in which the predetermined oxygen-storage-limits (ζlow, ζhigh) and/or lambda-limits are stored as a function of engine operating parameters.
7. A method for the control of an internal combustion engine (2) with a three-way-catalyst
TWC (4), comprising the following steps executed in parallel,
a) controlling the internal combustion engine (2) such that the level of oxygen stored
in the TWC (4) lies within predetermined oxygen-storage-limits (ζlow, ζhigh);
b) controlling the internal combustion engine (2) such that the lambda-value (λc) of the exhaust gas within or behind the TWC (4) lies within predetermined lambda-limits,
wherein a lambda-controller (7) for controlling the lambda-value (λ
c) is only operative if the level of oxygen stored in the TWC (4) lies within the predetermined
oxygen-storage-limits (ζ
low, ζ
high).
8. The method according to claim 7, characterized in that the level of oxygen stored in the TWC (4) is modelled as a function of engine operating
parameters.
9. The method according to claim 8, characterized in that the modelling is adapted based on a comparison between the modelled and the measured
value of an operating parameter.
1. Regelungssystem für einen Verbrennungsmotor (2) mit einem Dreiwegekatalysator TWC
(4), folgende Parallelanordnung aufweisend:
a) einen Sauerstoffspeicherregler (1), der geeignet ist, den Verbrennungsmotor (2)
derart zu regeln, dass die Menge von gespeichertem Sauerstoff in dem TWC (4) innerhalb
vorbestimmter Sauerstoffspeichergrenzen (ζniedrig, ζhoch) liegt;
b) einen Lambdaregler (7), der geeignet ist, den Verbrennungsmotor derart zu regeln,
dass der Lambdawert (λc) des Abgases innerhalb oder hinter dem TWC (4) innerhalb vorbestimmter Lambdagrenzen
liegt,
dadurch gekennzeichnet, dass der Lambdaregler (7) nur dann betriebsbereit ist, wenn die in dem TWC (4) gespeicherte
Menge an Sauerstoff innerhalb der vorbestimmten Sauerstoffspeichergrenzen (ζ
niedrig, ζ
hoch) liegt.
2. Regelungssystem nach Anspruch 1, dadurch gekennzeichnet, dass es einen HEGO-Sensor (5) zum Messen des Lambdawertes (λc) des Abgases innerhalb oder hinter dem TWC (4) aufweist.
3. Regelungssystem nach einem der Ansprüche 1 oder 2, dadurch gekennzeichnet, dass es ein Katalysatorregelmodell (6) aufweist, um die Menge (ζgeschätzt) von in dem TWC (4) gespeicherten Sauerstoff vorauszuberechnen.
4. Regelungssystem nach Anspruch 3, dadurch gekennzeichnet, dass das Katalysatorregelmodell (6) als Eingangssignal den Lambdawert (λe), den Massenstrom (mF) und/oder die Temperatur (T) des Abgases, das in den TWC (4) eintritt und/oder aus
ihm austritt, empfängt.
5. Regelungssystem nach Anspruch 3 oder 4, dadurch gekennzeichnet, dass das Katalysatorregelmodell (6) eine Anpassungseinheit aufweist, die fähig ist, das
Modell, basierend auf einem Vergleich zwischen den berechneten und den innerhalb oder
hinter dem TWC (4) gemessenen Lambdawerten (λc), abzugleichen.
6. Regelungssystem nach einem der Ansprüche 1 bis 5, dadurch gekennzeichnet, dass es einen Datenspeicher aufweist, in dem die vorbestimmten Sauerstoffspeichergrenzen
(ζniedrig, ζhoch) und/oder die Lambdagrenzen als eine Funktion der Motorbetriebsparameter gespeichert
sind.
7. Verfahren für die Regelung eines Verbrennungsmotors (2) mit einem Dreiwegekatalysator
TWC (4), die folgenden, parallel ausgeführten Schritte aufweisend,
a) Regeln des Verbrennungsmotors (2) derart, dass die Menge des in dem TWC (4) gespeicherten
Sauerstoffs innerhalb vorbestimmter Sauerstoffspeichergrenzen (ζniedrig, ζhoch) liegt;
b) Regeln des Verbrennungsmotors (2) derart, dass der Lambdawert (λc) des Abgases innerhalb oder hinter dem TWC (4) innerhalb der vorbestimmten Lambdagrenzen
liegt,
wobei ein Lambdaregler (7) zum Regeln der Lambdawerte (λ
c) nur betriebsbereit ist, wenn die Menge des in dem TWC (4) gespeicherten Sauerstoffs
innerhalb der vorbestimmten Sauerstoffspeichergrenzen (ζ
niedrig, ζ
hoch) liegt.
8. Verfahren nach Anspruch 7, dadurch gekennzeichnet, dass die Menge des in dem TWC (4) gespeicherten Sauerstoffs als eine Funktion von Motorbetriebsparametern
berechnet ist.
9. Verfahren nach Anspruch 8, dadurch gekennzeichnet, dass das Berechnen basierend auf einem Vergleich zwischen dem berechneten und dem gemessenen
Wert eines Betriebsparameters abgeglichen ist.
1. Système de commande d'un moteur à combustion interne (2) à catalyseur à trois voies
TWC (4), qui comprend l'agencement en parallèle de :
a) un contrôleur (1) de stockage d'oxygène adapté pour commander le moteur à combustion
interne (2) de telle sorte que le niveau d'oxygène stocké dans le TWC(4) reste à l'intérieur
de limites prédéterminées de stockage d'oxygène (ζbas, ζhaut),
b) un contrôleur de lambda (7) adapté pour commander le moteur à combustion interne
(2) de telle sorte que la valeur de lambda (λc) des gaz d'échappement à l'intérieur ou en aval du TWC (4) soit située à l'intérieur
des limites prédéterminées de lambda,
caractérisé en ce que le contrôleur de lambda (7) ne fonctionne que si le niveau d'oxygène stocké dans
le TWC (4) est situé à l'intérieur des limites prédéterminées de stockage d'oxygène
(ζ
bas, ζ
haut).
2. Système de contrôle selon la revendication 1, caractérisé en ce qu'il comprend un détecteur HEGO (5) qui mesure la valeur de lambda (λc) des gaz d'échappement à l'intérieur ou en aval du TWC (4).
3. Système de contrôle selon l'une des revendications 1 ou 2, caractérisé en ce qu'il comprend un modèle (6) de catalyseur qui prédit le niveau (ζest) d'oxygène stocké dans le TWC (4).
4. Système de contrôle selon la revendication 3, caractérisé en ce que le modèle (6) de catalyseur reçoit comme signal d'entrée la valeur de lambda (λe), le débit massique (mF) et/ou la température (T) des gaz d'échappement qui entrent et/ou sortent du TWC
(4).
5. Système de contrôle selon les revendications 3 ou 4, caractérisé en ce que le modèle (6) de catalyseur comprend une unité d'adaptation capable d'ajuster le
modèle en fonction d'une comparaison entre la valeur modélisée et la valeur mesure
de lambda (λe) à l'intérieur ou en aval du TWC (4).
6. Système de contrôle selon l'une des revendications 1 à 5, caractérisé en ce qu'il comprend une mémoire dans laquelle les limites prédéterminées de stockage d'oxygène
(ζbas, ζhaut) et/ou les limites de lambda sont conservées en fonction des paramètres de fonctionnement
du moteur.
7. Procédé de contrôle d'un moteur à combustion interne (2) doté d'un catalyseur à trois
voies TWC (4) et qui comprend les étapes suivantes, exécutées en parallèle :
a) commander le moteur à combustion interne (2) de telle sorte que le niveau d'oxygène
stocké dans le TWC (4) reste à l'intérieur des limites prédéterminées de stockage
d'oxygène (ζbas, ζhaut),
b) commander le moteur à combustion interne (2) de telle sorte que la valeur de lambda
(λc) des gaz d'échappement à l'intérieur ou en aval du TWC (4) soit située à l'intérieur
de limites prédéterminées de lambda,
un contrôleur de lambda (7) qui contrôle la valeur de lambda (λ
c) ne fonctionnant que si le niveau d'oxygène stocké dans le TWC (4) est situé à l'intérieur
des limites prédéterminées de stockage d'oxygène (ζ
bas, ζ
haut).
8. Procédé selon la revendication 7, caractérisé en ce que le niveau d'oxygène stocké dans le TWC (4) est modélisé en fonction de paramètres
de fonctionnement du moteur.
9. Procédé selon la revendication 8, caractérisé en ce que la modélisation est adaptée à partir d'une comparaison entre la valeur modélisée
et la valeur mesurée d'un paramètre de fonctionnement.