CA2493529A1

CA2493529A1 - Method and device for controlling the functioning of a nitrogen oxide trap for an internal combustion engine running on a lean mixture

Info

Publication number: CA2493529A1
Application number: CA002493529A
Authority: CA
Inventors: Stephane Cochet; Bernard Dionnet; Marc Daneau; Fabrice Gauvin; Karim Guenounou
Original assignee: Individual
Current assignee: Renault SAS
Priority date: 2002-07-31
Filing date: 2003-07-30
Publication date: 2004-02-12
Also published as: WO2004012846A2; KR20050030965A; FR2843044B1; AU2003281833A8; FR2843044A1; EP1525045A2; AU2003281833A1; US20060117738A1; WO2004012846A3; JP2005534855A

Abstract

Disclosed is a method according to which a first oxygen sensor (21) is disposed on the exhaust pipe upstream from a nitrogen oxide trap (18), and the development of a meaningful signal that is representative of the signal (3, 4) supplied by said sensor is monitored. A substantial increase (34, 44) of said meaningful signal, which is obtained following a variation (31, 41) resulting from the motor being switched from running on a lean mixture to running on a rich mixture, from a first plate (32, 42) having an essentially constant level is used as an indicator for controlling the end of the purge process. The invention applies to diesel engines.

Description

' CA 02493529 2005-O1-27 Method and device for control of the operation of a nitrogen oxides tra~~ for an internal combustion engine running on lean mixture.
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.
More particularly, it is intsnded for diesel engines, or in general for engines whose operation takes place under conditions of emission of exhaust gas with relatively low richness.
The use of catalysts of the iitrogen 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 ai: the engine outlet is increasing with the current attem~>ts 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 oper~ition with lean mixture, the richness at the intake is lowE~r than 1, the value corresponding to a stoichiometric mixture.
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 unbL.rden it, which is achieved by an operation known as purging, during which the nitrogen oxides content is re3uced. 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 uses upstream from the nitrogen oxides trap is greater tzan 1 and that the oxygen concentration is low, that is, such that the engine generates reducing agents such as HC, CO and H2, capable of reducing the nitrogen oxides stored on the substrate of the trap, by means o. the following reactions:
N02 + 2 H2 -> i~ N2 + 2 H20 N02 + 2 CO -> '~ N2 + 2 C02 (m/4 + n) N02 + CnHm -> (m/8 + n/a) N2 + n C02 + m/2 H20 Termination of the purging c~.n 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 tray, and there is the risk that either purging will be incomplete or that the phase of running on rich mixture will be impractically long.
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.

With these objectives in mini, 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 ;urging of the said nitrogen oxides trap is commanded periodically.
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 su:~stantially constant level, reached following a variation subsequent to a changeover of the engine from runiing on lean mixture to running on rich mixture, being used as the indicator to command the end of purging.
In the method according to tze 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 pr~.ctically as soon as the beginning of purging and at which the signal had previously substantially stabiliz:d. The inventors in fact observed that, under substan~ially constant conditions upstream from the nitrogen oxides trap during purging, the signal delivered by she oxygen sensor disposed downstream from the nitrogen oxides trap still underwent a meaningful evolution ~t the moment when the reduction of all stored nitrogen oxides was sufficiently complete.
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 i~ initiated, and that therefore this premature upward szoot obviously could not be used to indicate the end of reduction of the nitrogen oxides stored in the nitrogen oxi3es trap.
It is recalled here that larri~da 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 ~Tariations, but the signal that they are capable of delivering nevertheless varies abruptly if the evolution of the oxygen content is strong and fast.
For engines running on lean ~nixture, 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 ~ large excess of HC in the gases passing through the nitrogen oxides trap and arriving immediately at the downs~ream sensor, which itself reacts almost immediately :~y reaching saturation.
It was known beforehand, in she case of a gasoline engine running on lean mixture, teat the upward shoot of the signal of the sensor indicate. 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 t~ 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 cn the sensor while these reactions are occurring, an3 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.
In contrast to the foregoing, it became apparent to the inventors that, in the case cf 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 expo~iential, while it is linear for HC. Now, since the red~zcing power of H2 and of CO is greater than that of HC, th~~ purging reactions will therefore take place preferentially with these HZ and CO
reducing agents but, because of t:ze 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 a~ purging begins. The variation of the signal delivered by the sensor therefore no longer is indicative of the tonal reduction of the nitrogen oxides contained in the nitrogen oxides trap, but to the contrary is evident as soon as purging begins.
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.
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, anc then remained at this level as long as the engine continued to run on rich mixture. The inventors then imagiZed 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 aztomatically the return to running the engine on normal mixture, that is, on lean mixture.
One explanation of this phen~~menon is that, during the purging phase, the quantity o:. hydrogen downstream from the nitrogen oxides trap is :lose to zero, because the hydrogen upstream is consumed almost completely by the catalytic reactions, the reducing power of H2 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 H2 reducing agents will be found in increasing concentration downstream from the nitrogen oxides trap and simu=.taneously will favor the formation of HZ within the nitrogen oxides trap via various mechanisms, including in ~>articular:
H20 + CO - > HZ + C02 This formation of H2 was measured by gas-phase chromatography and continuous mass spectrometry.
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 cr the sensors of proportional type, such as the sensors known under the WO 2004/012$46 ~ PCTIFR2003/002421 name of UEGO~ sensor, which then ~~resent a signal in the form of the said second plateau s.ltuated at a level higher than the first plateau. Th~~ oxygen sensors of the lambda type and the oxygen sensor~~ of the proportional type are particularly sensitive t~~ hydrogen present. in the exhaust gases, and they exhibLt a large variation of their signal when the H2 concentr,~tion in the exhaust gases changes and therefore durin~~ the shift from the first to the second plateau.
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 engins to return to running on poor mixture, if necessary witz a certain time delay after detection of the variation ;roper.
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 tze 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.
In addition, with oxygen sensors of the same type ' CA 02493529 2005-O1-27 V'O 20041012846 PCT/FR2003/002421 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 Nhich is corrected as a function of the temperature of the sensitive element, the higher HZ concentration in the ga:~es downstream than in the gases upstream from the nitro~~en oxides trap is manifested by a higher signal for the sensor situated downstream than for the sensor si~:uated upstream when the reduction of the stored nitrogen oxides is sufficiently advanced.
As a function of the H2 concentration upstream, and in the case in which two lambda sensors are used, the signal of the upstream lambda sen:>or can be at the same level as the signal of the downstoeam lambda sensor at the second plateau.
Traditionally, the oxygen ser~sor or sensors will be chosen from among the sensors of the following type:
sensor of lambda type, proportion~.l oxygen sensor, nitrogen oxides detector, in whicr. the capability of measuring the oxygen concentration. is used.
In the case of use of a senscr upstream and of a sensor downstream from the nitrogEn oxides trap, it will also be possible for the two senscrs 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.

U stream sensor Downstream sensor Pro ortional sensor Lamb is sensor _ 02 fu xtion of the Nox Pro ortional sensor detector Lambda sensor Pro o rtional sensor Lambda sensor 02 fu action of the Nox detector 02 function of the Nox Lamb is sensor detector 02 function of the Nox Pro o rtional sensor detector where "02 function of the No:~ detector" denotes the use of the oxygen-concentration measuring function of a nitrogen oxides detector, as indicated in the foregoing.
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 time d or to use cells equivalent to that of the lambda sensor.
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.
According to a second embodiment, the increase of the meaningful signal is detected by applying filtering of the second derivative of the m=_aningful signal and observing the passage of the filtsred second derivative through zero in decreasing threshold.

According to a third embodiment, the increase of the meaningful signal is detected by raking 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.
According to a fourth embodivnent, the increase of the meaningful signal, far a lambda sensor, is detected by comparing the voltage value delivered by the sensor with a predetermined threshold.
A practical example of the i:ivention will now be described for a diesel engine.
Reference will be made to th~~ attached drawings, wherein:
- Fig. 1 is a schematic amd partial representation of the gas circuit of the said engine, - Fig. 2 is a graph that ;shows the curves representative of the signal of d:_fferent sensors disposed upstream and downstream ::rom the nitrogen oxides trap, and that illustrates the variation of the signal used as indicator of the end of purging.
On Fig. 1 there are schematically represented the elements of the engine through wh=.ch the gases pass, and there are seen therein, successively in the direction of flow of the gases:
- inlet 11 for air arriving j:rom the air filter, - suction part 12 of a turbooompressor, - intake manifold 13, - the upper part of cylinder 14, equipped with an injector 15, - exhaust tubes 16 and the e:thaust part 17 of the turbocompressor, - nitrogen oxides trap 18, - the exhaust line, equipped with a particle filter 19.
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 ~~alculating unit 23, which itself is connected to an e:igine-control unit 24.
Sensors 21 and 22 are, for e:cample, proportional or lambda sensors.
The graph of Fig. 2 shows thc~ results of test measurements performed with different types of sensors as a function of the time from initi<~tion of purging. On the ordinate, the left scale represeni:s the value of the signal delivered by a sensor, and the right scale represents the quantity of CO dow~istream from the nitrogen oxides trap (for trace 6;.
Line 3 represents the signal delivered by a sensor of lambda type disposed downstreall 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 arr:_val of HZ at the sensor and is therefore indicative of thf: end of purging, before the signal restabilizes at a second plateau 33.

' CA 02493529 2005-O1-27 Line 4 represents the signal delivered by a sensor of proportional type disposed dow:lstream 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, repre;~entative of the arrival of HZ at the sensor and is therefore indicative of the end of purging, before the signal~resi:abilizes at a second plateau 43.
In comparison, line 5 represents the signal delivered by a sensor of proporticraal 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 gages. 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 mill 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.
It will also be noted that the level of plateau 42 is situated below plateau 52, the ~.ifference 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.
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 whicz, marked by the strong increase 61 at the moment when th= 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 adJanced disappearance thereof.
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 incr'ase 71, corresponding to the increase of the slope of the signal obtained from the sensor.
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.
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 V~'O 20041012846 PCT/FR2003/002421 general to all engines that normally run on lean mixture.

Claims

1. A method for control of the operation of a nitrogen oxides trap (18) for an internal combustion engine running on lean mixture, wherein purging of the said nitrogen oxides trap is commanded periodically, characterized in that a first oxygen sensor (21) is disposed in the exhaust pipe downstream from the nitrogen oxides trap (18), and the evolution of a meaningful signal representative of the signal (3, 4) delivered by this sensor is observed, a substantial increase (34, 44) of this meaningful signal from a first plateau (32, 42) of substantially constant level, reached following a variation (31, 41) 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.

2. A method according to claim 1, characterized in that there is additionally used a second oxygen sensor (22) disposed upstream from the nitrogen oxides trap (18), in order to deliver a reference signal (5), relative to which the evolution of the signal delivered by the first sensor is compared in order to deliver the said meaningful signal.

3. A method according to claim 1 or 2, characterized in that the increase of the meaningful signal is detected by applying filtering of the first derivative (7) of the meaningful signal and by comparing the filtered first derivative with a predetermined threshold.

4. A method according to claim 1 or 2, characterized in that the increase of the meaningful signal is detected by applying filtering of the second derivative (8) of the meaningful signal and observing the passage of the filtered second derivative through zero in decreasing threshold.

5. A method according to claim 1 or 2, characterized in that 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.

6. A method according to claim 1 or 2, characterized in that 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.

7. A method according to claim 1, characterized in that the oxygen sensor or sensors (21, 22) is or are chosen from among the sensors of the following type:
sensor of lambda type, proportional oxygen sensor, nitrogen oxides detector, in which the oxygen-concentration measuring function is used.

8. A method according to claim 7 in combination with claim 2, characterized in that the two sensors (21, 22) are of different types.

9. A device for control of the operation of a nitrogen oxides trap (18) for an internal combustion engine running on lean mixture, for use of the method according to one of claims 1 to 7, the engine being equipped with an exhaust line (16) containing a nitrogen oxides trap (18), characterized in that it is provided with an oxygen sensor (21) disposed on the exhaust line downstream from the nitrogen oxides trap, and with calculating means (23) for determining a substantial increase of a meaningful signal representative of the signal (3, 4) delivered by the said sensor from a first plateau (32, 42) of substantially constant level, reached following initiation of a purging operation, and using the said increase as indicator to command the end of purging.

10. A device according to claim 9, characterized in that it is provided with a second oxygen sensor (22) disposed upstream from the nitrogen oxides trap (18) and connected to the said calculating means (23) in order to deliver a reference signal thereto.