U.S. patent application number 10/522724 was filed with the patent office on 2006-06-08 for method and device for controlling the functioning of a nitrogen oxide trap for an internal combustion engine running on a lean mixture.
This patent application is currently assigned to RENAULT S.A.A.. Invention is credited to Stephane Cochet, Marc Daneau, Bernard Dionnet.
Application Number | 20060117738 10/522724 |
Document ID | / |
Family ID | 30129564 |
Filed Date | 2006-06-08 |
United States Patent
Application |
20060117738 |
Kind Code |
A1 |
Cochet; Stephane ; et
al. |
June 8, 2006 |
Method and device for controlling the functioning of a nitrogen
oxide trap for an internal combustion engine running on a lean
mixture
Abstract
A method in which a first oxygen sensor is disposed on an
exhaust pipe upstream from a nitrogen oxide trap, and development
of a meaningful signal representative of the signal supplied by the
sensor is monitored. A substantial increase of the meaningful
signal, which is obtained following a variation resulting from a
motor being switched from running on a lean mixture to running on a
rich mixture, from a first plate having an essentially constant
level is used as an indicator to control an end of a purge process.
The method can be applied to diesel engines.
Inventors: |
Cochet; Stephane;
(Versailles, FR) ; Dionnet; Bernard;
(Morigny-Champigny, FR) ; Daneau; Marc; (Boulogne
Billancourt, FR) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
RENAULT S.A.A.
BOULOGNE BILLANCOURT
FR
|
Family ID: |
30129564 |
Appl. No.: |
10/522724 |
Filed: |
July 30, 2003 |
PCT Filed: |
July 30, 2003 |
PCT NO: |
PCT/FR03/02421 |
371 Date: |
August 25, 2005 |
Current U.S.
Class: |
60/285 ; 60/295;
60/301 |
Current CPC
Class: |
F01N 2250/14 20130101;
F02D 41/1456 20130101; F01N 13/009 20140601; F01N 3/0821 20130101;
F01N 2570/14 20130101; F01N 3/0842 20130101; F01N 2250/12 20130101;
F02B 37/00 20130101; F02D 41/1441 20130101; Y02A 50/20 20180101;
F02D 41/1454 20130101; Y02A 50/2344 20180101; F01N 3/0814 20130101;
F02D 2041/1432 20130101; B01D 53/9495 20130101; F01N 2430/06
20130101; F02D 41/0275 20130101 |
Class at
Publication: |
060/285 ;
060/295; 060/301 |
International
Class: |
F01N 3/00 20060101
F01N003/00; F01N 3/10 20060101 F01N003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 31, 2002 |
FR |
02/09712 |
Claims
1-10. (canceled)
11. A method for control of operation of a nitrogen oxides trap for
an internal combustion engine running on a lean mixture, wherein
purging of the nitrogen oxides trap is commanded periodically, and
a first oxygen sensor is disposed in an exhaust pipe downstream
from the nitrogen oxides trap, the method comprising: observing
evolution of a meaningful signal representative of a signal
delivered by the first oxygen sensor; using an increase of the
meaningful signal from a first plateau of substantially constant
level, reached following a variation subsequent to a changeover of
the engine from running on a lean mixture to running on a rich
mixture, as an indicator to command an end of purging.
12. A method according to claim 11, wherein a second oxygen sensor
disposed upstream from the nitrogen oxides trap is additionally
used to deliver a reference signal, relative to which the evolution
of the signal delivered by the first oxygen sensor is compared to
deliver the meaningful signal.
13. A method according to claim 11, wherein the increase of the
meaningful signal is detected by applying filtering of a first
derivative of the meaningful signal and by comparing the filtered
first derivative with a predetermined threshold.
14. A method according to claim 12, wherein the increase of the
meaningful signal is detected by applying filtering of a first
derivative of the meaningful signal and by comparing the filtered
first derivative with a predetermined threshold.
15. A method according to claim 11, wherein the increase of the
meaningful signal is detected by applying filtering of a second
derivative of the meaningful signal and observing passage of the
filtered second derivative through zero in decreasing
threshold.
16. A method according to claim 12, wherein the increase of the
meaningful signal is detected by applying filtering of a second
derivative of the meaningful signal and observing passage of the
filtered second derivative through zero in decreasing
threshold.
17. A method according to claim 11, wherein the increase of the
meaningful signal is detected by taking a difference between an
instantaneous value of the meaningful signal and a sliding mean of
the meaningful signal, and by comparing the difference with a
threshold.
18. A method according to claim 12, wherein the increase of the
meaningful signal is detected by taking a difference between an
instantaneous value of the meaningful signal and a sliding mean of
the meaningful signal, and by comparing the difference with a
threshold.
19. A method according to claim 11, wherein the increase of the
meaningful signal, for a lambda sensor, is detected by comparing a
voltage value delivered by the first oxygen sensor with a
predetermined threshold.
20. A method according to claim 12, wherein the increase of the
meaningful signal, for a lambda sensor, is detected by comparing a
voltage value delivered by the first oxygen sensor with a
predetermined threshold.
21. A method according to claim 11, wherein the first oxygen sensor
is chosen from among sensors of a sensor of lambda type,
proportional oxygen sensor, nitrogen oxides detector, in which the
oxygen-concentration measuring function is used.
22. A method according to claim 12, wherein the first oxygen sensor
is chosen from among sensors of a sensor of lambda type,
proportional oxygen sensor, nitrogen oxides detector, in which the
oxygen-concentration measuring function is used.
23. A method according to claim 22, wherein the first and second
oxygen sensors are of different types.
24. A device for control of an operation of a nitrogen oxides trap
for an internal combustion engine running on a lean mixture, for
use of the method according to claim 11, the engine being equipped
with an exhaust line containing a nitrogen oxides trap, the device
comprising: a first oxygen sensor disposed on an exhaust line
downstream from the nitrogen oxides trap; and calculating means for
determining an increase of a meaningful signal representative of
the signal delivered by the first oxygen sensor from a first
plateau of substantially constant level, reached following
initiation of a purging operation, and using the increase as an
indicator to command an end of purging.
25. A device according to claim 24, further comprising a second
oxygen sensor disposed upstream from the nitrogen oxides trap and
connected to the calculating means to deliver a reference signal
thereto.
Description
[0001] The present invention relates to a method and a device for
control of the operation of a nitrogen oxides trap for an internal
combustion engine running on lean mixture.
[0002] More particularly, it is intended for diesel engines, or in
general for engines whose operation takes place under conditions of
emission of exhaust gas with relatively low richness.
[0003] The use of catalysts of the nitrogen oxides trap type, also
commonly known as "Nox Trap", is already known in diesel engines.
It corresponds to the desire to avoid discharge of nitrogen oxides
in the exhaust gases, since the emission of nitrogen oxides at the
engine outlet is increasing with the current attempts to run the
engines on lean mixture, that is under conditions of excess oxygen
relative to the fuel and therefore excess air. In other words, in
this type of operation with lean mixture, the richness at the
intake is lower than 1, the value corresponding to a stoichiometric
mixture.
[0004] During operation of the engine, a nitrogen oxides trap
continuously captures the nitrogen oxides contained in the exhaust
gases produced by the said engine. It is therefore necessary to
regenerate the nitrogen oxides trap periodically in order to
unburden it, which is achieved by an operation known as purging,
during which the nitrogen oxides content is reduced. It is known to
perform this purging periodically, temporarily commanding the
richness of the engine to be increased, in such a way that the
richness of the exhaust gases upstream from the nitrogen oxides
trap is greater than 1 and that the oxygen concentration is low,
that is, such that the engine generates reducing agents such as HC,
CO and H.sub.2, capable of reducing the nitrogen oxides stored on
the substrate of the trap, by means of the following reactions:
NO2+2 H2->1/2N2+2 H2O NO2+2 CO->1/2N2+2 CO2
(m/4+n)NO2+CnHm->(m/8+n/2)N2+n CO2+m/2 H2O
[0005] Termination of the purging can be commanded, for example, at
the end of a predetermined time after the engine was changed over
to running on rich mixture. However, that does not make any
allowance for the real burden of the nitrogen oxides trap, and
there is the risk that either purging will be incomplete or that
the phase of running on rich mixture will be impractically
long.
[0006] The object of the present invention is to overcome this
problem and in particular to provide a method and means with which
it is possible, in optimal manner, to command a stop to purging of
a catalyst of the nitrogen oxides trap type disposed in the exhaust
line of an internal combustion engine that normally runs on lean
mixture.
[0007] With these objectives in mind, the object of the invention
is a method for control of the operation of a nitrogen oxides trap
for an internal combustion engine running on lean mixture, wherein
purging of the said nitrogen oxides trap is commanded
periodically.
[0008] According to the invention, this method is characterized in
that a first oxygen sensor is disposed in the exhaust pipe
downstream from the nitrogen oxides trap, and the evolution of a
meaningful signal representative of the signal delivered by this
sensor is observed, a substantial increase of this meaningful
signal from a first plateau of substantially constant level,
reached following a variation subsequent to a changeover of the
engine from running on lean mixture to running on rich mixture,
being used as the indicator to command the end of purging.
[0009] In the method according to the invention, the command to end
purging of the nitrogen oxides trap is therefore achieved in
response to a notable evolution of the meaningful signal
representative of the condition of the oxygen sensor disposed
downstream from the trap, this evolution comprising a marked
increase of the signal beyond a first plateau reached practically
as soon as the beginning of purging and at which the signal had
previously substantially stabilized. The inventors in fact observed
that, under substantially constant conditions upstream from the
nitrogen oxides trap during purging, the signal delivered by the
oxygen sensor disposed downstream from the nitrogen oxides trap
still underwent a meaningful evolution at the moment when the
reduction of all stored nitrogen oxides was sufficiently
complete.
[0010] One difficulty encountered was actually that, in the case of
engines running on lean mixture, such as diesel engines, the signal
of an oxygen sensor disposed downstream from the nitrogen oxides
trap shoots up almost immediately, as soon as purging is initiated,
and that therefore this premature upward shoot obviously could not
be used to indicate the end of reduction of the nitrogen oxides
stored in the nitrogen oxides trap.
[0011] It is recalled here that lambda sensors normally deliver a
signal that is practically binary as a function of whether or not
oxygen is being detected. Proportional sensors are certainly more
suitable for measuring a range of concentrations or progressive
variations, but the signal that they are capable of delivering
nevertheless varies abruptly if the evolution of the oxygen content
is strong and fast.
[0012] For engines running on lean mixture, the initiation of
purging, which takes place by changeover to a substantially richer
mode of operation in order to provide reducing elements capable of
reacting with the stored nitrogen oxides, leads to a large excess
of HC in the gases passing through the nitrogen oxides trap and
arriving immediately at the downstream sensor, which itself reacts
almost immediately by reaching saturation.
[0013] It was known beforehand, in the case of a gasoline engine
running on lean mixture, that the upward shoot of the signal of the
sensor indicated the disappearance of oxygen at the outlet of the
nitrogen oxides trap, which therefore could be interpreted as the
end of purging, that is, the arrival again of a reducing mixture at
the sensor, whereas previously, during purging, the reducing agents
originating from the engine were being consumed by the reduction
reactions used for purging. In fact, in this case, the proportion
of CO to HC in the exhaust gases is relatively high, but, since the
reducing power of CO is much stronger than that of HC, a kind of
equilibrium is established between the aforesaid reduction
reactions, leading to a substantially equivalent participation of
all reducing agents delivered by the engine in purging of the
nitrogen oxides trap. As a result, the gaseous mixture exiting the
nitrogen oxides trap then remains without effect on the sensor
while these reactions are occurring, and it is only at the end of
the reduction reactions that the reducing agents are then present
in excess at the outlet of the nitrogen oxides trap, causing the
sensor signal to shoot up, since it is then truly representative of
the end of purging.
[0014] In contrast to the foregoing, it became apparent to the
inventors that, in the case of the engines considered by the
present invention, operating with a richness on the order of 1 to
1.1, the upward shoot of the signal takes place almost as soon as
purging begins following the relatively massive arrival of HC at
the sensor disposed downstream from the nitrogen oxides trap. An
explanation of this phenomenon, which does not occur in the case of
operation with greater richness, such as 1.2 to 1.4, appears to lie
in the fact that the HC/CO ratio is higher in the exhaust gases in
the case of operation with lower richness. In fact, the evolution
of CO content as a function of richness is exponential, while it is
linear for HC. Now, since the reducing power of H.sub.2 and of CO
is greater than that of HC, the purging reactions will therefore
take place preferentially with these H.sub.2 and CO reducing agents
but, because of the relative excess of HC, the equilibrium of
reactions mentioned in the foregoing no longer is established, thus
leading to the presence of unreacted HC at the outlet of the
nitrogen oxides trap practically as soon as purging begins. The
variation of the signal delivered by the sensor therefore no longer
is indicative of the total reduction of the nitrogen oxides
contained in the nitrogen oxides trap, but to the contrary is
evident as soon as purging begins.
[0015] The inventors therefore searched for another means of
detecting this end of purging. They discovered that, surprisingly,
since in particular the lambda sensors are considered to deliver a
quasi-binary signal, this signal delivered by an oxygen sensor
underwent a new variation starting from a substantially constant
plateau at which the arrival of the HC had caused the signal to
shoot up as soon as purging began.
[0016] An analysis of this phenomenon led to the observation that,
after the signal arrived in this way at the level of the first
plateau and was maintained there during purging as long as the
aforesaid reduction reactions did not treat the majority of the
nitrogen oxides stored on the catalyst substrate of the nitrogen
oxides trap, this signal shifted to a second plateau of higher
level, substantially at the moment of the end of the said reduction
reactions, and then remained at this level as long as the engine
continued to run on rich mixture. The inventors then imagined
taking advantage of this evolution of the shift of the signal from
the first to the second plateau as an indicator of the end of
purging and thus for commanding automatically the return to running
the engine on normal mixture, that is, on lean mixture.
[0017] One explanation of this phenomenon is that, during the
purging phase, the quantity of hydrogen downstream from the
nitrogen oxides trap is close to zero, because the hydrogen
upstream is consumed almost completely by the catalytic reactions,
the reducing power of H.sub.2 being predominant, as already
indicated hereinabove. When the reduction reactions are terminated
by virtue of the small residual quantity of nitrogen oxides stored
in the nitrogen oxides trap, the HC, CO and H.sub.2 reducing agents
will be found in increasing concentration downstream from the
nitrogen oxides trap and simultaneously will favor the formation of
H.sub.2 within the nitrogen oxides trap via various mechanisms,
including in particular: H.sub.2O+CO->H.sub.2+CO.sub.2
[0018] This formation of H.sub.2 was measured by gas-phase
chromatography and continuous mass spectrometry.
[0019] This modification of the concentration of reducing agents
downstream from the nitrogen oxides trap influences the signal of
oxygen sensors of all-or-nothing type, such as the lambda sensors
or the sensors of proportional type, such as the sensors known
under the name of UEGO.RTM. sensor, which then present a signal in
the form of the said second plateau situated at a level higher than
the first plateau. The oxygen sensors of the lambda type and the
oxygen sensors of the proportional type are particularly sensitive
to hydrogen present in the exhaust gases, and they exhibit a large
variation of their signal when the H2 concentration in the exhaust
gases changes and therefore during the shift from the first to the
second plateau.
[0020] This particular feature therefore made it possible to take
advantage of the variation of the signal from its first plateau to
detect the end of purging and consequently to command the engine to
return to running on poor mixture, if necessary with a certain time
delay after detection of the variation proper.
[0021] According to a particular arrangement of the invention,
there is additionally used a second oxygen sensor disposed upstream
from the nitrogen oxides trap, in order to deliver a reference
signal relative to which the evolution of the signal delivered by
the first sensor is compared in order to deliver the said
meaningful signal. The use of this reference signal, which can be
influenced by variations of the operating conditions of the engine
independent of those programmed for the purging operation proper,
therefore makes it possible, by comparison, to reliably detect the
variation that is indicative of the end of the reduction reactions
in the nitrogen oxides trap.
[0022] In addition, with oxygen sensors of the same type upstream
and downstream from the nitrogen oxides trap, for example two
lambda sensors or two proportional sensors, the sensitive element
of which is temperature-controlled or else the signal of which is
corrected as a function of the temperature of the sensitive
element, the higher H.sub.2 concentration in the gases downstream
than in the gases upstream from the nitrogen oxides trap is
manifested by a higher signal for the sensor situated downstream
than for the sensor situated upstream when the reduction of the
stored nitrogen oxides is sufficiently advanced.
[0023] As a function of the H.sub.2 concentration upstream, and in
the case in which two lambda sensors are used, the signal of the
upstream lambda sensor can be at the same level as the signal of
the downstream lambda sensor at the second plateau.
[0024] Traditionally, the oxygen sensor or sensors will be chosen
from among the sensors of the following type: sensor of lambda
type, proportional oxygen sensor, nitrogen oxides detector, in
which the capability of measuring the oxygen concentration is
used.
[0025] In the case of use of a sensor upstream and of a sensor
downstream from the nitrogen oxides trap, it will also be possible
for the two sensors to be of different types, and particular, it
will then be possible to use, for the purposes of the invention,
sensors that may already be installed in the exhaust line to
perform other functions. In particular, it will be possible to use
the different combinations listed in the following table.
TABLE-US-00001 Upstream sensor Downstream sensor Proportional
sensor Lambda sensor Proportional sensor O2 function of the Nox
detector Lambda sensor Proportional sensor Lambda sensor O2
function of the Nox detector O2 function of the Nox detector Lambda
sensor O2 function of the Nox detector Proportional sensor
[0026] where "O2 function of the Nox detector" denotes the use of
the oxygen-concentration measuring function of a nitrogen oxides
detector, as indicated in the foregoing.
[0027] When two oxygen sensors of different nature are used, it is
necessary either to introduce a transfer function between the
sensor signals (taking into account any differences in response
times) or to use cells equivalent to that of the lambda sensor.
[0028] According to a first embodiment, the increase of the
meaningful signal is detected by applying filtering of the first
derivative of the meaningful signal and by comparing the filtered
first derivative with a predetermined threshold.
[0029] According to a second embodiment, the increase of the
meaningful signal is detected by applying filtering of the second
derivative of the meaningful signal and observing the passage of
the filtered second derivative through zero in decreasing
threshold.
[0030] According to a third embodiment, the increase of the
meaningful signal is detected by taking the difference between the
instantaneous value of the meaningful signal and a sliding mean of
the said signal, and by comparing this difference with a
threshold.
[0031] According to a fourth embodiment, the increase of the
meaningful signal, for a lambda sensor, is detected by comparing
the voltage value delivered by the sensor with a predetermined
threshold.
[0032] A practical example of the invention will now be described
for a diesel engine.
[0033] Reference will be made to the attached drawings,
wherein:
[0034] FIG. 1 is a schematic and partial representation of the gas
circuit of the said engine,
[0035] FIG. 2 is a graph that shows the curves representative of
the signal of different sensors disposed upstream and downstream
from the nitrogen oxides trap, and that illustrates the variation
of the signal used as indicator of the end of purging.
[0036] On FIG. 1 there are schematically represented the elements
of the engine through which the gases pass, and there are seen
therein, successively in the direction of flow of the gases: [0037]
inlet 11 for air arriving from the air filter, [0038] suction part
12 of a turbocompressor, [0039] intake manifold 13, [0040] the
upper part of cylinder 14, equipped with an injector 15, [0041]
exhaust tubes 16 and the exhaust part 17 of the turbocompressor,
[0042] nitrogen oxides trap 18, [0043] the exhaust line, equipped
with a particle filter 19.
[0044] A first oxygen sensor 21 is mounted downstream from the
nitrogen oxides trap, and a second oxygen sensor 22 is mounted
upstream from the nitrogen oxides trap, the two sensors being
connected to a calculating unit 23, which itself is connected to an
engine-control unit 24.
[0045] Sensors 21 and 22 are, for example, proportional or lambda
sensors.
[0046] The graph of FIG. 2 shows the results of test measurements
performed with different types of sensors as a function of the time
from initiation of purging. On the ordinate, the left scale
represents the value of the signal delivered by a sensor, and the
right scale represents the quantity of CO downstream from the
nitrogen oxides trap (for trace 6).
[0047] Line 3 represents the signal delivered by a sensor of lambda
type disposed downstream from the nitrogen oxides trap. On the line
there is clearly seen the upward shoot 31 of the sensor as soon as
purging begins, and the first plateau 32 which remains
substantially stable during purging, then the marked variation 34
of the signal, representative of the arrival of H.sub.2 at the
sensor and is therefore indicative of the end of purging, before
the signal restabilizes at a second plateau 33.
[0048] Line 4 represents the signal delivered by a sensor of
proportional type disposed downstream from the nitrogen oxides
trap. On the line there is clearly seen the upward shoot 41 of the
sensor as soon as purging begins, and the first plateau 42 which
remains substantially stable during purging, then the marked
variation 44 of the signal, representative of the arrival of
H.sub.2 at the sensor and is therefore indicative of the end of
purging, before the signal restabilizes at a second plateau 43.
[0049] In comparison, line 5 represents the signal delivered by a
sensor of proportional type disposed upstream from the nitrogen
oxides trap. It is clearly seen that this signal also undergoes an
abrupt rise 51 at the moment of initiation of purging, due to the
influx of reducing agents in the exhaust gases. In contrast, the
signal then remains constant on a plateau 52, as long as the engine
is maintained under constant conditions of running on rich mixture.
In the event of any perturbation whatsoever of these conditions
during purging, that perturbation will correspondingly affect both
the upstream sensor and the sensor situated downstream from the
nitrogen oxides trap, and it will be possible to use this signal
delivered by the upstream sensor as a reference for evaluating, by
comparison, the variations of the signal of the downstream sensor,
truly indicative of the end of purging.
[0050] It will also be noted that the level of plateau 42 is
situated below plateau 52, the difference corresponding to the
fraction of the reducing agents used to reduce the nitrogen oxides
stored in the filter. In contrast, when purging is terminated, the
signal of the downstream sensor shifts above that of the upstream
sensor, clearly illustrating the arrival at the sensor of the
hydrogen formed in the nitrogen oxides trap by the aforesaid
reaction mechanism.
[0051] On this graph there was also represented a line 6
representing the evolution of the CO content downstream from the
nitrogen oxides trap, measured in experimental manner, and the
evolution of which, marked by the strong increase 61 at the moment
when the sensor signals shift from the first to the second plateau,
clearly corroborates the end of consumption of the said CO for
reducing the nitrogen oxides stored in the nitrogen oxides trap,
and therefore the advanced disappearance thereof.
[0052] As an example of how the signal obtained from the sensors is
processed, there was also represented in the graph the line 7,
which represents the filtered first derivative of the signal, and
illustrates the possibility of detecting the end of purging by
comparing this signal with a predetermined threshold, crossed by
the said derivative during its abrupt increase 71, corresponding to
the increase of the slope of the signal obtained from the
sensor.
[0053] Similarly, there was also represented the line 8, which
represents the second derivative, whose passage through zero in
decreasing threshold 81 can also be used as information indicative
of the end of purging.
[0054] The invention is not limited to the examples described
hereinabove. It is intended more particularly for diesel engines,
but it could also be applied in general to all engines that
normally run on lean mixture.
* * * * *