U.S. patent application number 11/169350 was filed with the patent office on 2006-01-05 for method and device for adjustment of a fuel/air ratio for an internal combustion engine.
Invention is credited to Stefan Buchner, Alf Degen, Ekkehard Pott, Ralf Rothkegel.
Application Number | 20060000200 11/169350 |
Document ID | / |
Family ID | 32683496 |
Filed Date | 2006-01-05 |
United States Patent
Application |
20060000200 |
Kind Code |
A1 |
Pott; Ekkehard ; et
al. |
January 5, 2006 |
Method and device for adjustment of a fuel/air ratio for an
internal combustion engine
Abstract
In a method for adjusting the fuel/air ratio in an internal
combustion engine (1) comprising a converter (2) which is
associated therewith, a composition of waste gas in the waste gas
wing (3, 8) of the internal combustion engine (1) is detected by
means of sensors (4, 5) and output signals from at least one of the
sensors (4, 5) are used for producing a control signal in order to
influence the fuel/air ratio. The fuel/air ratio is switched back
and forth between a lean operating state with surplus oxygen and a
rich operating state with an oxygen deficit by means of a
characteristic line of the control signal. The characteristic line
of the control signal is adapted to a current converter state. A
characteristic curve contour of the characteristic line is adjusted
according to the addition and/or desorption of an oxidation agent
in the converter (2). The invention also relates to a device for
carrying out said method.
Inventors: |
Pott; Ekkehard; (Gifhorn,
DE) ; Degen; Alf; (Meinersen, DE) ; Buchner;
Stefan; (Braunschweig, DE) ; Rothkegel; Ralf;
(Wolfsburg, DE) |
Correspondence
Address: |
Baker Botts LLP
98 San Jacinto Blvd
1500 San Jacinto Ctr
Austin
TX
78701--403
US
|
Family ID: |
32683496 |
Appl. No.: |
11/169350 |
Filed: |
June 29, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP03/14968 |
Dec 30, 2003 |
|
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|
11169350 |
Jun 29, 2005 |
|
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Current U.S.
Class: |
60/285 |
Current CPC
Class: |
F01N 2430/06 20130101;
Y02T 10/22 20130101; F02D 41/1441 20130101; F02D 41/1456 20130101;
F02D 41/1495 20130101; F02D 41/1477 20130101; F02D 41/0295
20130101; Y02T 10/12 20130101; F02D 2041/389 20130101 |
Class at
Publication: |
060/285 |
International
Class: |
F01N 3/00 20060101
F01N003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 30, 2002 |
DE |
102 61 751.1 |
Mar 12, 2003 |
DE |
103 10 672.3 |
Claims
1. A method for adjustment of a fuel/air ratio for an internal
combustion engine with an associated catalyst, comprising the steps
of: determining an exhaust composition in an exhaust system of the
internal combustion engine by means of sensors, generating a
control signal to influence the fuel/air ratio as a function of
output signals of at least one of the sensors, and making by means
of a characteristic curve of the control signal a switch back and
forth between an operating state with oxygen excess and an
operating state with oxygen deficiency of the catalyst, wherein a
shape of the characteristic curve is adjusted as a function of an
oxygen and/or NO.sub.x addition and/or desorption capability of the
catalyst.
2. A method according to claim 1, wherein a course of a transition
from operating state to another and/or a course of the
characteristic curve within an operating state is adjusted as a
function of oxygen and/or NO.sub.x addition and/or desorption
capability of the catalyst.
3. A method according to claim 1, wherein the characteristic curve
of the control signal is adjusted as a function of a catalyst
temperature.
4. A method according to claim 1, wherein the characteristic curve
of the control signal is adjusted as a function of the degree of
aging of the catalyst.
5. A method according to claim 1, wherein adjustment of the
characteristic curve of the control signal occurs as a function of
the operating parameters of an internal combustion engine.
6. A method according to claim 1, wherein the characteristic curve
of the control signal is adjusted unsymmetrically to a stipulated
lambda value over a time range that includes at least several
periods of the control signal.
7. A method according to claim 1, wherein the characteristic curve
of the control signal is a sawtooth.
8. A method according to claim 1, wherein the subsequent operating
states are adjusted with different residence time of the control
signal.
9. A method according to claim 1, wherein the subsequent operating
states are adjusted with different amplitude of the control
signal.
10. A method according to claim 1, wherein the characteristic curve
of the control signal is nonlinear at least in a region.
11. A method according to claim 10, wherein the characteristic
curve of the control signal becomes leaner or richer
degressively.
12. A method according to claim 11, wherein the characteristic of
the control signal initially becomes leaner around a stipulated
amount or richer and then is degressively guided in the direction
.lamda.=1.00.
13. A method according to claim 1, wherein the characteristic curve
of the control signal is a rectangular curve with different
amplitudes and/or residence times in the adjusted operating
states.
14. A method according to claim 1, wherein during fuel cutoff in
the overrun or idle of the internal combustion engine, the internal
combustion engine is operated more in the rich operating state than
in the lean operating state.
15. A method according to claim 1, wherein in a catalyst the
control signal is adjusted so that increased incorporation of
oxygen and/or NO.sub.x in the catalyst occurs temporarily.
16. A method according to claim 1, wherein in a catalyst after a
stipulated operating time the control signal is adjusted so that a
phase with increased lean operation follows a phase with at least
two periods with mostly rich operation.
17. A method according to claim 1, wherein before reaching an
operating temperature of the catalyst the characteristic of the
control signal deviates from the characteristic after surpassing
the operating temperature.
18. A method according to claim 1, wherein in a catalyst at almost
operating temperature, the characteristic curve of the control
signal has a sawtooth trend before reaching a stipulated operating
temperature.
19. A method according to claim 1, wherein the catalyst state
and/or a state of the sensor upstream and/or downstream of the
catalyst is determined from the control signal.
20. A device for adjustment of a fuel/air ratio for an internal
combustion engine with an associated catalyst, comprising: exhaust
composition sensors, a control unit for generating a control signal
to influence the fuel/air ratio as a function of output signals of
at least one of the sensors, said control signal comprising a
characteristic curve for switching back and forth between an
operating state with oxygen excess and an operating state with
oxygen deficiency of the catalyst, and means for adjusting a form
of a characteristic curve as a function of oxygen and/or NO.sub.x
addition and/or desorption in the catalyst.
21. A device according to claim 20, wherein a sensor is arranged in
the exhaust system of the internal combustion engine upstream and
downstream of the catalyst.
22. A device according to claim 21, wherein the sensor upstream of
catalyst is a broadband lambda probe with a constant
characteristic.
23. A device according to claim 22, wherein the sensor upstream of
the catalyst is a two-point lambda probe with a transfer
characteristic.
24. A device according to claim 21, wherein the sensor downstream
of the catalyst is a two-point lambda probe with a transfer
characteristic.
25. A device according to claim 21, wherein the sensor downstream
of the catalyst is a broadband lambda probe with a constant
characteristic.
26. A device according to claim 20, wherein the catalyst is a
three-way catalyst.
27. A device according to claim 26, wherein the catalyst has a
noble metal content of less than 60 g/ft.sup.3, especially less
than 40 g/ft.sup.3, preferably less than 30 g/ft.sup.3, optimally
less than 20 g/ft.sup.3, ideally less than 10 g/ft.sup.3.
28. A device according to claim 20, wherein the catalyst is an
NO.sub.x storage catalyst.
29. A device according to claim 28, wherein the catalyst has a
noble metal content of less than 80 g/ft.sup.3, especially less
than 60 g/ft.sup.3.
30. A device according to claim 20, wherein the internal combustion
engine is a directly injected internal combustion engine capable of
layered charging.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of copending
International Application No. PCT/EP2003/014968 filed Dec. 30, 2003
which designates the United States, and claims priority to German
application no. 102 61 751.1 filed Dec. 30, 2002 and German
application no. 103 10 672.3 filed Mar. 12, 2003.
TECHNICAL FIELD
[0002] The invention concerns a method and device for adjustment of
a fuel/air ratio for an internal combustion engine.
BACKGROUND
[0003] A catalyst arranged in the exhaust system of an internal
combustion engine is ordinarily used to clean the exhaust of an
internal combustion engine. This converts harmful components, like
hydrocarbons CH, carbon monoxide CO and oxides of nitrogen NO.sub.x
essentially to nontoxic gases. It is critical to the so-called
degree of conversion of the catalyst that the oxygen content of the
exhaust lie within optimal values. For a so-called three-way
catalyst these optimal values lie in a narrow range around the
value corresponding to a fuel/air mixture of .lamda.=1. In order to
be able to maintain this narrow range, it is customary to regulate
the fuel/air ratio for an internal combustion engine by means of
oxygen sensors arranged in the exhaust system of an internal
combustion engine.
[0004] A method for lambda control for internal combustion engine
with a downstream catalyst is known from Unexamined Patent
Application DE 40 24 212 A1 in which the oxygen fractions of the
exhaust of the internal combustion engine are recorded by oxygen
sensors upstream and downstream of the catalyst. In stipulated
operating ranges a control signal with controllable amplitude is
generated by coupling in an outside signal with controllable
amplitude. With increasing catalyst aging the amplitude is reduced.
The functional state of the catalyst in the exhaust system of the
internal combustion engine can be determined with the method by
means of lambda regulation and the time for replacement of an aged
catalyst determined.
[0005] A method for adjustment of the fuel/air ratio for an
internal combustion engine with a downstream catalyst is known from
Unexamined Patent Application DE 43 37 793 A1 in which the oxygen
fractions in the exhaust of the internal combustion engine are
determined by oxygen sensors upstream and downstream of the
catalyst. Both sensors influence regulation of the fuel/air ratio.
It is initially determined with an amplitude evaluation whether the
catalyst has already reached a certain degree of aging. This actual
control quantity is issued by the sensor upstream of the catalyst.
A switch is then made to frequency evaluation or frequency
regulation in which the catalyst yields the actual control quantity
downstream of the catalyst. Such evaluations are sensitive per se,
but have a relatively strong influence on the operating behavior of
the internal combustion engine. This is avoided by only switching
to frequency evaluation when the catalyst has already reached a
certain state of aging. With increasing operating time the oxygen
storage capability of the catalyst declines. The control frequency
therefore increases with increasing catalyst aging so that lambda
regulation is adjusted to the state of aging of the catalyst. As
soon as the determined control frequency downstream of the catalyst
is higher than the frequency threshold, aging of the catalyst can
be reliably recognized and the catalyst replaced.
SUMMARY
[0006] The object of the invention is to improve the method for
adjusting the fuel/air ratio for an internal combustion engine with
a downstream catalyst according to the prior art and permit greater
dynamics, as well as to devise an apparatus for execution of the
method.
[0007] This object can be solved by a method for adjustment of a
fuel/air ratio for an internal combustion engine with an associated
catalyst, comprising the steps of determining an exhaust
composition in an exhaust system of the internal combustion engine
by means of sensors, generating a control signal to influence the
fuel/air ratio as a function of output signals of at least one of
the sensors, and making by means of a characteristic curve of the
control signal a switch back and forth between an operating state
with oxygen excess and an operating state with oxygen deficiency of
the catalyst, wherein a shape of the characteristic curve is
adjusted as a function of an oxygen and/or NO.sub.x addition and/or
desorption capability of the catalyst.
[0008] A course of a transition from operating state to another
and/or a course of the characteristic curve within an operating
state can be adjusted as a function of oxygen and/or NO.sub.x
addition and/or desorption capability of the catalyst. The
characteristic curve of the control signal can be adjusted as a
function of a catalyst temperature. The characteristic curve of the
control signal can also be adjusted as a function of the degree of
aging of the catalyst. Adjustment of the characteristic curve of
the control signal may occur as a function of the operating
parameters of an internal combustion engine. The characteristic
curve of the control signal can be adjusted unsymmetrically to a
stipulated lambda value over a time range that includes at least
several periods of the control signal. The characteristic curve of
the control signal may be a sawtooth. The subsequent operating
states can be adjusted with different residence time of the control
signal. The subsequent operating states can be adjusted with
different amplitude of the control signal. The characteristic curve
of the control signal may be nonlinear at least in a region. The
characteristic curve of the control signal may become leaner or
richer degressively. The characteristic of the control signal may
initially become leaner around a stipulated amount or richer and
then is degressively guided in the direction .lamda.=1.00. The
characteristic curve of the control signal can be a rectangular
curve with different amplitudes and/or residence times in the
adjusted operating states. During fuel cutoff in the overrun or
idle of the internal combustion engine, the internal combustion
engine may be operated more in the rich operating state than in the
lean operating state. In a catalyst the control signal may be
adjusted so that increased incorporation of oxygen and/or NO.sub.x
in the catalyst occurs temporarily. In a catalyst after a
stipulated operating time the control signal can be adjusted so
that a phase with increased lean operation follows a phase with at
least two periods with mostly rich operation. Before reaching an
operating temperature of the catalyst the characteristic of the
control signal may deviate from the characteristic after surpassing
the operating temperature. In a catalyst at almost operating
temperature, the characteristic curve of the control signal may
have a sawtooth trend before reaching a stipulated operating
temperature. The catalyst state and/or a state of the sensor
upstream and/or downstream of the catalyst can be determined from
the control signal.
[0009] The object can also be achieved by a device for adjustment
of a fuel/air ratio for an internal combustion engine with an
associated catalyst, comprising exhaust composition sensors, a
control unit for generating a control signal to influence the
fuel/air ratio as a function of output signals of at least one of
the sensors, said control signal comprising a characteristic curve
for switching back and forth between an operating state with oxygen
excess and an operating state with oxygen deficiency of the
catalyst, and means for adjusting a form of a characteristic curve
as a function of oxygen and/or NO.sub.x addition and/or desorption
in the catalyst.
[0010] A sensor can be arranged in the exhaust system of the
internal combustion engine upstream and downstream of the catalyst.
The sensor upstream of catalyst may be a broadband lambda probe
with a constant characteristic. The sensor upstream of the catalyst
can also be a two-point lambda probe with a transfer
characteristic. The sensor downstream of the catalyst may be a
two-point lambda probe with a transfer characteristic. The sensor
downstream of the catalyst may also be a broadband lambda probe
with a constant characteristic. The catalyst can be a three-way
catalyst. The catalyst may have a noble metal content of less than
60 g/ft.sup.3, especially less than 40 g/ft.sup.3, preferably less
than 30 g/ft.sup.3, optimally less than 20 g/ft.sup.3, ideally less
than 10 g/ft.sup.3. The catalyst can be an NO.sub.x storage
catalyst. The catalyst may have a noble metal content of less than
80 g/ft.sup.3, especially less than 60 g/ft.sup.3. The internal
combustion engine can be a directly injected internal combustion
engine capable of layered charging.
[0011] One advantage of the method is that the time trend of the
reference value of lambda value of the exhaust upstream of a
catalyst connected after the engine is automatically adjusted to
the operating states of the catalyst in which conversion is
otherwise not optimum. This leads to better utilization of the
catalyst and to increased reliability in maintaining emission
values.
[0012] Another advantage of the invention is that because of the
improved dynamics of the exhaust system in a vehicle in which the
device according to the invention was implemented the driving
dynamics are approved.
[0013] In addition, the catalyst during its lifetime is brought to
favorable operating ranges and operated with high efficiency so
that in comparison with ordinarily regulated catalysts noble metals
of the catalyst can be saved and/or the catalyst itself reduced in
size. This saves cost and resources.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The invention is further explained by means of drawings, in
which the figures show:
[0015] FIG. 1 shows a schematic view of a preferred device for
execution of the method according to the invention,
[0016] FIG. 2 shows a characteristic curve of a control signal
according to the prior art for a three-way catalyst with new
catalyst (2a) and with aged catalyst (2b),
[0017] FIG. 3 shows a first preferred characteristic curve of a
control signal according to the invention with a rectangular trend
and different transfer height,
[0018] FIG. 4 shows a second preferred characteristic curve of a
control signal according to the invention with a sawtooth
trend,
[0019] FIG. 5 shows a third preferred characteristic curve of a
control signal according to the invention with a degressive
trend,
[0020] FIG. 6 shows a fourth preferred characteristic curve of a
control signal according to the invention with also a degressive
trend,
[0021] FIG. 7 shows a fifth preferred characteristic curve of a
control signal according to the invention with a rectangular trend
and progressive runout,
[0022] FIG. 8 shows a sixth preferred characteristic curve of a
control signal according to the invention with a rectangular trend
and a sawtooth runout.
DETAILED DESCRIPTION
[0023] The invention is particularly suited for catalysts in which
the air fraction fed to the internal combustion engine of a
fuel/air mixture is adjusted by means of a control signal that is
periodically set between a minimal value and a maximal value of the
air fraction and switched back and forth between a rich operating
state with oxygen deficiency and a lean operating state with oxygen
excess.
[0024] The invention is particularly favorable for a three-way
catalyst in which oxygen and/or NO.sub.x are periodically
introduced as oxidizers for the catalyst and desorbed and in which
a control signal of a lambda control deviates essentially
periodically around the lambda value .lamda.=1. In the lean
operating state the oxygen supply in the exhaust is sufficient to
oxidize its HC and CO fractions, whereas in the rich operating
state NO.sub.x fractions in the exhaust as oxidizers oxidize the HC
and CO fractions present. A common control strategy for a three-way
catalyst proposes a lambda regulation in which a .lamda. probe is
exposed to a control signal with constant frequency. In the lean
operating state when .lamda.>1 oxygen is introduced to the
catalyst 2; in the rich operating state with .lamda.<1 this
oxygen is consumed for oxidation processes.
[0025] However, it is also possible to use the invention NO.sub.x
storage catalyst that can be operated at higher lambda values in a
three-way catalyst. Aged storage catalysts can also be operated at
lower lambda values around .lamda.=1.
[0026] FIG. 1 schematically depicts a preferred device for
execution of the method according to the invention. A catalyst 2 is
connected in the exhaust 3 after the internal combustion engine 1.
The internal combustion engine 1 is supplied in the usual manner
with a fuel/air mixture via means not further shown; air supply
preferably occurs via an intake line 7. In the exhaust line 3
upstream of catalyst 2 a sensor 4 is arranged, which detects the
composition of the exhaust. The sensor 4 is preferably an oxygen
sensor that detects the oxygen content in the exhaust. The sensor 4
is preferably a broadband lambda probe with a constant control
characteristic. Downstream of catalyst 2 another sensor is arranged
in the exhaust line 8 that can detect the composition of the
exhaust purified in catalyst 2. Preferably, an oxygen sensor is
also used here, with particular preference a two-point lambda probe
with a transfer characteristic. The invention also includes devices
with more than one downstream catalyst 2.
[0027] In principle, ordinary lambda probes are suitable with
sensors 4, 5, like broadband lambda probes, two-point lambda probes
or NO.sub.x sensor with lambda probe function. As an alternative, a
two-point lambda probe can also be used upstream of catalyst 2
and/or a broadband lambda probe downstream of catalyst 2 has
sensors 4, 5. It is also conceivable to determine the lambda value
upstream of catalyst 2 from other types of measured quantities,
like injected amount of fuel and drawn in amount of air.
[0028] The oxygen storage capability of catalyst 2 varies over the
lifetime of the catalyst 2. The characteristic of a lambda probe,
especially a broadband lambda probe can also vary. This can be
compensated by adjusting the frequency of the control signal to the
state of aging. Expediently, sensor 4 is exposed to a control
signal upstream of catalyst 2. Sensor 5 downstream of catalyst 2
reports to sensor 4 upstream as soon as breakthrough of rich
exhaust or lean exhaust is observed behind catalyst 2. As long as
lean exhaust is available up to sensor 5 downstream of catalyst 2
oxygen breakthrough is recognized via the internal combustion
engine 1. A switch is made to the rich operating state of catalyst
2 until breakthrough of the rich component is observed. A switch is
then made back to the lean operating state and the sequence is
repeated.
[0029] With increasing age the oxygen storage capability of
catalyst 2 diminishes, breakthroughs occur more quickly and the
control frequency rises. The lambda control is therefore adapted to
the state of aging of catalyst. Such regulation is also referred to
as natural frequency regulation.
[0030] The sensors 4, 5 are connected to a control device 6 that
receives their signals and sends them to evaluation. This control
device 6 is expediently a component of an ordinary engine control
device used for operation of the internal combustion engine 1. In
this control device 6 or via this device operating parameters of
the internal combustion engine 1 or a vehicle driven by the
internal combustion engine 1 are available. These operating
parameters are preferably entered as maps in a corresponding
storage medium. Such operating parameters include exhaust
temperature upstream of catalyst 2 and/or in catalyst 2, exhaust
temperature downstream of the catalyst, oxygen storage capability
of the catalyst 2, exhaust flow rate, speed of the internal
combustion engine 1, exhaust recirculation rate, position of a
camshaft disk, charge movement flap, ignition time and/or charge
pressure and the like. Information concerning the operating
parameters can be linked to the sensor signals and control
therefore conducted as a function of the operating parameters. This
is indicated by arrows on control device 6. Individual operating
parameters or different operating parameters can be used in
combination with each other.
[0031] Two characteristic curves of an ordinary control signal
according to the prior art for a fresh three-way catalyst (FIG. 2a)
and an aged three-way catalyst (2b) are shown in FIG. 2. The
frequency of the control signal of fresh catalyst 2 is distinctly
smaller than that of the aged catalyst 2. However, otherwise the
characteristic curve of the control signal is unchanged, since the
characteristic curve shape and especially the amplitude are
retained.
[0032] Means are provided according to the invention in order to
adjust a characteristic curve of the control signal to an actual
catalyst state so that a characteristic curve shape of the
characteristic curve is adjustable as a function of addition and/or
desorption of an oxidizer in catalyst 2. Such a characteristic
curve shape can involve a transition from one operating state to
another and/or a trend of the characteristic curve within an
operating state of catalyst 2. The oxidizer can be oxygen or
NO.sub.x.
[0033] In this case the frequency of the control signal is not
followed simply as described in the prior art according to the
state of aging of catalyst 2, but the characteristic curve shape of
the characteristic curve is varied by varying the amplitude and/or
characteristic curve, especially a flank steepness, switching point
and/or trend in an operating state. This adjustment occurs within a
control cycle and can vary with increasing operating time of
catalyst 2. The aging behavior of catalyst 2 can be different in
rich and lean operating states so that consideration of the
different behavior in the two operating states permits more
efficient utilization of catalyst 2 via a corresponding adjustment
of the control signal as a function of the catalyst state.
[0034] Depending on the state of aging of catalyst 2 the amplitude
of the characteristic curve for a rich and lean operating state of
the catalyst can be adjusted. This is shown in FIG. 3. An abrupt
control signal with variable amplitude is shown there. In addition,
the frequency can also be modulated. By varying the amplitude the
system can be accelerated. If the sensor 5 downstream of catalyst 2
establishes a strongly depleted exhaust, it can rapidly be adjusted
by stronger enrichment. During over-enrichment it can again be
rapidly depleted. The system can therefore reach equilibrium more
quickly. The amplitudes for the transition from the rich operating
state to the lean operating state and from the lean operating state
to the rich operating state can be different.
[0035] Such a control strategy is advantageous for catalysts that
have already been used for some time but are still useable over a
longer time. Here it is favorable to operate over several control
periods more strongly in the rich region, followed by a phase with
mostly lean fractions and then again richer fractions and to repeat
this. Because of this, increased oxygen storage in the catalyst 2
can be temporarily reached up to the limit of the regeneration
capability of catalyst 2.
[0036] A trend according to FIG. 3 can also be very advantageous
between internal combustion engine 1 operated with thrust operated
with fuel cutoff in the overrun. In this state, for example on a
gradient, no fuel is temporarily fed to the internal combustion
engine 1 and the internal combustion engine 1 operates on idle. The
exhaust quickly becomes lean. It is favorable here to adjust the
control signal so that the internal combustion engine is initially
exposed to a fuel/air mixture that over several periods on a time
average causes more rich fractions in the exhaust, which is
recognizable by the larger amplitudes in the rich operating state.
The internal combustion engine 1 is then operated in the lean
range. This permits improved dynamics of the system and better
driving dynamics of a vehicle operated in this way.
[0037] Over a time region that includes at least several periods of
the control signal it can therefore be advantageous to adjust the
amplitudes of the characteristic curve of the control signal
unsymmetrically to a stipulated lambda value.
[0038] According to a favorable modification of the invention the
characteristic curve of the control signal can be sawtooth. This is
shown in FIG. 4. The transitions between a lean operating state to
a rich operating state therefore do not occur abruptly, as in the
previous example, but the transitions occur more smoothly with a
finite slope of the flanks. This trend of the control signal is
suitable for catalyst 2 that has not reached or has still not
reached the optimum temperature range, especially in the phase with
average temperature between a cold start and the sought operating
temperature. It was found that the catalyst 2 can reach its optimal
temperature range for normal operation more quickly by means of the
sawtooth trend of the control signal. The flanks of the control
signal can then have different slopes in terms of amount as well as
different amplitudes.
[0039] It can prescribed that the characteristic curve of the
control signal is a rectangular curve or a different characteristic
curve with different amplitude and/or residence time in the
corresponding operating states so that the percentage of rich
operating states and lean operating states can be adjusted as a
function of needs to the actual state of catalyst 2.
[0040] It is also possible that adjust consecutive operating states
with different duration depending on how dependent the addition
process and/or desorption processes of the oxidizer in catalyst 2
are on the operating parameters of the internal combustion engine
and/or the lifetime of catalyst 2.
[0041] A control characteristic curve is shown in FIG. 5 that is
nonlinear and has a degressive trend. A transition from one
operating state to another occurs quickly with a relatively steep
flank, whereupon the characteristic curve is slightly rising to a
maximum or minimum value. The control signal can also have a
progressive trend.
[0042] A control characteristic curve is shown in FIG. 6 that is
nonlinear and has a progressive trend. A transition from one
operating state occurs initially quickly, but then with a
relatively flat flank.
[0043] FIGS. 7 and 8 show examples of control signals, in which
different curve forms are superimposed. The transitions between the
operating states are abrupt but in the operating state the lean
and/or fat fractions in the exhaust still increase nonlinearly or
linearly. There are also additional overlaps and combinations of
curve forms of the control signal that occur in succession that can
be adjusted as a function of need.
[0044] With increasing age the deep storage of oxygen and/or
NO.sub.x in catalyst 2 deteriorates so that the desired catalytic
processes can no longer occur efficiently. The behavior of the
catalyst 2 in lean operating states can be different than in rich
operating states. Variation in modulation of the characteristic
curve of the control signal therefore permits adjustment to these
boundary conditions with a simultaneous increase in efficiency of
the catalytic processes. This is advantageous to maintain emission
limits.
[0045] It is particularly expedient to conduct the adjustment of
the characteristic curve of the control signal as a function of
operating parameters in internal combustion engine 1. This can
occur via maps of operating parameters, as already described in
FIG. 1. It is therefore considered that the behavior of catalyst 2
is strongly influenced by the operating parameters of the internal
combustion engine 1. The rate of conversion changes sharply with
exhaust temperature. The catalyst 2 is exposed to pollutants with a
high exhaust flow rate so that in the extreme case the purification
efficiency of catalyst 2 can decline. If exhaust recirculation is
varied, the richness of the fuel/air mixture changes. If the amount
of recycled exhaust rises, the NO.sub.x content in the exhaust
drops. The ignition point influences the system in similar fashion
to exhaust recirculation. By influencing the combustion trend the
pollutant and oxygen concentration change at the same lambda value.
With low crude emissions or a shift to more readily convertible
pollutants (for example more CO, less CH.sub.4) the requirements on
accuracy of lambda control diminish. Such influences of the
operating parameters can be considered by corresponding adjustment
of the characteristic curve of the control signal so that
maintenance of the emission standard is ensured over broad
operating ranges of the internal combustion engine 1.
[0046] In addition to ensuring the emission standard, in an
advantageous embodiment of the invention the catalyst state and/or
state of the sensor 4, 5, especially sensor 5 downstream from
catalyst 2 can be determined from a change in the characteristic
curve of the control signal.
[0047] In addition to increased reliability in maintain in emission
values, the invention also permits a reduction in noble metal
content of catalyst 2. The catalyst 2 is mostly operated in regions
in which conversion is improved. Because of this the catalyst
volume can be correspondingly reduced and/or the noble metal
compound of the catalyst can be reduced in order to achieve the
same efficiency as in an ordinary control. It is possible to reduce
the noble metal content and/or the catalyst volume during use of
the method according to the invention without surpassing the
pollutant emissions forming without use of the method by at least
10%, especially by at least 20%. In particular, the catalyst 2 in
the case of an NO.sub.x storage catalyst has a noble metal content
of less 80 g/ft.sup.3, especially less than 60 g/ft.sup.3. In a
three-way catalyst a noble metal content of less than 60
g/ft.sup.3, especially less than 40 g/ft.sup.3, preferably less
than 30 g/ft.sup.3, optimally less than 20 g/ft.sup.3, ideally less
than 10 g/ft.sup.3 is provided.
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