WO2004012846A2

WO2004012846A2 - 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: WO2004012846A2
Application number: PCT/FR2003/002421
Authority: WO
Inventors: Stéphane Cochet; Bernard Dionnet; Marc Daneau; Fabrice Gauvin; Karim Guenounou
Original assignee: Renault S.A.S.
Priority date: 2002-07-31
Filing date: 2003-07-30
Publication date: 2004-02-12
Also published as: AU2003281833A1; AU2003281833A8; KR20050030965A; FR2843044A1; CA2493529A1; WO2004012846A3; FR2843044B1; JP2005534855A; EP1525045A2; US20060117738A1

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

Method and device for managing the operation of a nitrogen oxide trap for an internal combustion engine operating in a lean mixture.

The present invention relates to a method and a device for managing the operation of a nitrogen oxide trap for a combustion engine operating in a lean mixture.

It is more particularly intended for diesel engines, or, in general, engines whose operation is carried out with an exhaust gas emission regime having a relatively low richness.

The use of catalysts of the nitrogen oxide trap type, also commonly called "Nox Trap", on diesel engines is already known. It corresponds to the wish to avoid the rejection of nitrogen oxides in the exhaust gases, the emission of nitrogen oxides at the outlet of the engine being all the more important that one is currently trying to operate. engines in a lean mixture, that is to say in conditions of excess oxygen relative to the fuel and therefore in excess of air. In other words, in this type of lean mixture operation, the intake richness is less than 1, a value corresponding to the stoichiometric mixture.

During engine operation, a nitrogen oxide trap continuously captures the nitrogen oxides contained in the exhaust gases produced by said engine. It is therefore necessary to regenerate periodically the nitrogen oxide trap to discharge it, which is done by a so-called purge operation, during which the nitrogen oxides are reduced. It is known to perform this purging periodically, temporarily controlling an increase in the richness of the engine, such that the richness of the exhaust gases upstream of the nitrogen oxide trap is greater than 1 and the oxygen concentration low, that is to say that the engine generates reducers, such as HC, CO and H ₂ , capable of reducing the nitrogen oxides stored on the substrate of the trap, according to the following reactions:

N02 + 2 H2 -> ^ N2 + 2 H20

N02 + 2 CO -> 7 ₂ N2 + 2 C02

(m / 4 + n) N02 + CnHm -> (m / 8 + n / 2) N2 + n C02 + m / 2 H20

The end of the purge can be controlled for example after a predetermined time from the passage of the engine in operation in rich mixture. However, this does not then take into account the actual state of charge of the nitrogen oxide trap, and there is a risk either of having an incomplete purge, or of an unnecessarily long rich mixture operating phase.

The present invention aims to solve this problem and aims in particular to provide a method and means making it possible to optimally control the stopping of the purges of a catalyst of the nitrogen oxide trap type placed on the line. exhaust from an internal combustion engine normally operating in a lean mixture.

With these objectives in view, the subject of the invention is a method for managing the operation of a nitrogen oxide trap for an internal combustion engine operating in a lean mixture, according to which a purge of said trap is periodically controlled. nitrogen oxides.

According to the invention, this process is characterized in that there is a _ι first oxygen sensor on the exhaust duct downstream of the nitrogen oxide trap, and the evolution of a significant signal representative of the signal supplied by this probe, a significant increase in this significant signal from a first plateau of substantially constant level, obtained following a variation following a change in the engine from operation in lean mixture to operation in rich mixture, being used as an indicator to control the end of the purge.

In the method according to the invention, the command to end purging of the nitrogen oxide trap is therefore carried out in response to a significant change in the significant signal representative of the state of the oxygen sensor placed downstream of the trap, this evolution consisting in a marked increase in the signal beyond a first plateau reached practically from the start of the purge and where the signal had previously been substantially stabilized. The inventors have in fact found that, at substantially constant conditions upstream of the nitrogen oxide trap at During the purge, the signal supplied by the oxygen sensor placed downstream of the nitrogen oxide trap was still undergoing a significant evolution at the time when the reduction of all the stored nitrogen oxides was sufficiently complete.

A difficulty which arose was in fact that, in the case of engines operating in a lean mixture, such as diesel engines, the signal from an oxygen sensor placed downstream of the nitrogen oxide trap switches almost immediately from the start purge and that this early changeover could therefore obviously not be exploited to signify the end of the reduction of nitrogen oxides stored in the nitrogen oxide trap.

It is recalled here that lambda probes normally provide a practically binary signal depending on the detection or not of oxygen. Proportional probes are certainly better able to measure staggered concentrations or gradual variations, but the signal they can provide nevertheless varies suddenly if the evolution of the oxygen content is strong and rapid.

For engines operating in lean mixture, the initiation of a purge, which takes the form of switching to a substantially richer operating mode to provide reducing elements capable of reacting with the stored nitrogen oxides, leads to a large excess of HC in the gases which pass through the nitrogen oxide trap and arrive immediately on the downstream probe, which it reacts almost immediately, reaching saturation.

It was previously known, in the case of a gasoline engine operating in lean mixture, that the switching of the signal from the probe signaled the disappearance of oxygen at the outlet of the nitrogen oxide trap, which could therefore be interpreted as the end of the purge, that is to say again the arrival on the probe of a reducing mixture, whereas previously, during the purging, the reducers coming from the engine were consumed by the reactions of. reduction used for purging. In fact, in this case, the proportion of CO with respect to HC in the exhaust gases is relatively high but the reducing power of CO being stronger than that of HC, a kind of equilibrium is established between the reactions aforementioned reduction, leading to a substantially equivalent participation of all the reducers provided by the engine in the purging of the nitrogen oxide trap. As a result, the gas mixture leaving the nitrogen oxide trap remains without effect on the probe as long as these reactions occur, and it is only at the end of the reduction reactions that the reducers are found. then in excess at the outlet of the nitrogen oxide trap, causing the probe signal to toggle, truly representative then of the end of purging.

Contrary to this, it has appeared to the inventors that, in the case of the motors targeted by the present invention, operating with a richness of the order of 1 to 1.1, the signal switching occurs practically from the start of the purge following the relatively massive arrival of HC on the probe placed downstream of the nitrogen oxide trap. An explanation for this phenomenon, which does not occur in the case of an operation with a higher richness, of 1.2 to 1.4 for example, seems to lie in the fact that the HC / CO ratio is higher in exhaust gases when operating with a lower richness. In fact, the evolution of the CO content as a function of the richness is exponential, whereas it is linear for the HCs. Since the reducing power of H ₂ and CO is stronger than that of HC, the purge reactions will therefore preferably take place with these reducing agents H ₂ and CO, but, due to the relative excess of HC, l he balance of reactions mentioned above is no longer achieved, which leads to the presence of HC, which has not reacted, at the outlet of the nitrogen oxide trap practically from the start of the purge. The variation in the signal supplied by the probe is therefore no longer significant for the total reduction of the nitrogen oxides contained in the nitrogen oxide trap, but appears on the contrary from the start of the purging.

The inventors then looked for another means of detecting this end of purging. They discovered that, surprisingly since in particular lambda probes are considered to provide a quasi-binary signal, this signal supplied by an oxygen probe underwent a new variation from the substantially constant plateau where the arrival of HC had toggles the signal at the start of the purge. An analysis of this phenomenon has shown that, after the signal has thus reached the level of the first plateau, and that it has remained there during the purge until the abovementioned reduction reactions have treated the majority of the nitrogen oxides stored on the catalyst substrate of the nitrogen oxide trap, this signal passed to a second plateau, of higher level, substantially at the end of the said reduction reactions, and then remained at this level as long as the engine continued to run in a rich mixture. The inventors then imagined using this change in the passage of the signal from the first to the second plate as an indicator for the end of purging, and therefore for automatically controlling the return to operation of the engine in normal mixing, that is to say in lean mixing .

One explanation for this phenomenon is that, during the purging phase, the quantity of hydrogen downstream of the nitrogen oxide trap is close to zero because the upstream hydrogen is almost completely consumed by catalytic reactions, the power H ₂ reducer being preponderant as already indicated above. When the reduction reactions end due to the small residual quantity of nitrogen oxides stored in the nitrogen oxide trap, the reducing agents HC, CO and H ₂ will be found in increasing concentration downstream of the oxide trap nitrogen and at the same time promote the formation of H ₂ within the nitrogen oxide trap according to various mechanisms including in particular: H ₂ 0 + CO -> H ₂ + C0 ₂

This H ₂ formation was measured by gas chromatography and continuous mass spectrometry.

This change in the concentration of reducing agents downstream of the nitrogen oxide trap influences the signal of the oxygen probes of the all or nothing type, such as the lambda probes, or of the proportional probe type, such as the probes known under the name of UEGO ^® probe, which then present a signal in the form of said second tray located at a level higher than the first tray. Oxygen sensors of the lambda type and oxygen sensors of the proportional type are particularly sensitive to the hydrogen present in the exhaust gases and show a significant change in their signal when the concentration of H2 in the exhaust gases changes and so when switching from the first to the second tray.

This particularity therefore made it possible to use the variation of the signal from its first plate to detect the end of purging and consequently control the return of the engine operation in lean mixture, possibly with a certain time delay after the detection of the variation properly speaking. called.

According to a particular arrangement of the invention, a second oxygen sensor is used in addition placed upstream of the nitrogen oxide trap, to provide a reference signal by report to which the evolution of the signal provided by the first probe is compared to provide said significant signal. The use of this reference signal, which can be influenced by variations in the operating conditions of the engine independent of those programmed for the purging operation proper, therefore allows by comparison to reliably detect the significant variation at the end of the reduction reactions in the nitrogen oxide trap.

Furthermore, with the same type of oxygen sensors upstream and downstream of the nitrogen oxide trap, for example two lambda sensors or two sensors

15 proportional, whose sensitive element is regulated in temperature or whose signal is corrected as a function of the temperature of the sensitive element, the concentration of H ₂ higher in the gases downstream than in the gases upstream of the trap nitrogen oxides

20 results in a higher signal for the downstream probe than for the upstream probe when the reduction of the stored nitrogen oxides is sufficiently advanced.

7. Depending on the concentration of H ₂ upstream and in the case ^where two lambda probes are used, the signal of the upstream lambda probe may be at the same level signal, the downstream lambda probe on the second tray.

30

The one or ^'probe (s) for oxygen will typically be chosen from the following types of sensors: lambda probe type, proportional probe oxygen, nitrogen oxide sensor which uses the oxygen concentration measurement function.

In the case of the use of a probe upstream and a probe downstream of the nitrogen oxide trap, the two probes may also be of different types, and in particular it will thus be possible to use for the purposes of the invention of probes possibly already installed on the exhaust line to perform other functions. We can in particular use the different combinations appearing in the following table.

where "function 02 of the Nox sensor" means the use of the oxygen concentration measurement function of a nitrogen oxide sensor, as indicated previously.

When using two different oxygen probes, you must introduce either a transfer function between the signals of the probes

(with possible consideration of different response times) or use cells equivalent to that of the lambda probe.

According to a first embodiment, the increase in the significant signal is detected in filtering the first derivative of the significant signal and comparing the filtered first derivative to a predetermined threshold.

According to a second embodiment, the increase in the significant signal is detected by filtering the second derivative of the significant signal and observing the passage to zero in decreasing threshold of the filtered second derivative.

According to a third embodiment, the increase in the significant signal is detected by making the difference between the instantaneous value of the significant signal and a sliding average of said signal, and by comparing this difference with a threshold.

According to a fourth embodiment, the increase in the significant signal, for a lambda probe, is detected by comparing the voltage value delivered by the probe to a predetermined threshold.

We will now describe an exemplary implementation of the invention on a diesel engine.

Reference will be made to the appended drawings in which: FIG. 1 is a schematic and partial representation of the gas circuit of said engine,

- Figure 2 is a graph showing the representative curves of the signal from different probes placed upstream and downstream of the oxide trap nitrogen, and illustrating the signal variation used as the end of purge indicator.

FIG. 1 schematically shows the elements of the engine through which the gases pass, and we can see there, successively according to the direction of gas flow:

- the air inlet 11, coming from the air filter, - the suction part 12 of a turbocharger,

- the intake manifold 13,

- the top of the cylinder 14 fitted with an injector 15, - the exhaust pipes 16 and the exhaust part 17 of the turbocharger,

- the nitrogen oxide trap 18,

- the exhaust line, equipped with a particle filter 19.

A first oxygen probe 21 is mounted downstream of the nitrogen oxide trap, and a second oxygen probe 22 is mounted upstream of the nitrogen oxide trap, the two probes being connected to a computing unit 23, it - even connected to a pilo aye unit of the 24 motor.

The probes 21 and 22 are for example proportional, or lambda probes.

The graph in FIG. 2 shows the results of test measurements carried out with different types of probes, as a function of the time from the start of a purge. On the ordinate, the scale on the left represents the value of the signal supplied by a splitting machine, and the scale on the right represents the quantity of CO downstream of the nitrogen oxide trap (for trace 6) 5

Line 3 represents the signal supplied by a lambda type probe placed downstream of the nitrogen oxide trap. We can clearly see on the plot the tilting 31 of the probe from the start of the purge, and the first

10. plate 32 which remains substantially stable during the purge, then there marked variation of the signal 34, representative of the arrival of H ₂ on the probe, and therefore significant of the end of the purge, before the signal stabilizes again on a second plateau

15 33.

Line 4 represents the signal supplied by a proportional type probe placed downstream of the nitrogen oxide trap. We can clearly see on the plot the 0 tilt 41 of the probe from the start of the purge, and the first plate 42 which remains substantially stable during the purge, then the marked variation of the signal 44, representative of the arrival of H ₂ on the probe, and therefore significant of the end of the purge, before the signal stabilizes again on a second plate 43.

In comparison, trace 5 represents the signal supplied by a proportional type probe placed 0 upstream of the nitrogen oxide trap. It is clear that this signal also undergoes a sudden rise 51, at the time of initiation of the purge, due to the influx of reducers in the exhaust gases. On the other hand, the signal then remains constant on a level 52, as long that the engine is maintained in a constant operating mode in rich mixture. If any disruption of this scheme during purging, ^'this disturbance correspondingly affect the upstream probe as the probe located downstream of the trap nitrogen oxides, and this signal provided by the upstream sensor may be a reference to evaluate by comparison the variations in the signal from the downstream probe that are really significant at the end of the purge.

It will also be noted that the level of the plate 42 is located below the plate 52, the difference corresponding to the share of reducers used to reduce the nitrogen oxides stored in the filter. On the other hand, when the purging is finished, the signal from the downstream probe passes over that of the upstream probe, which clearly illustrates the arrival on the probe of the hydrogen formed in the nitrogen oxide trap according to the aforementioned reaction mechanism.

Also shown on this ^"graph a plot 6 showing the variation in the downstream CO content of the nitrogen oxide trap, experimentally measured, and whose evolution, characterized by the sharp increase ^'61 at the time of the passage of the signals from the probes from the first to the second level, corroborates well the end of the use of said ÇO to reduce the nitrogen oxides stored in the nitrogen oxide trap, therefore the advanced disappearance of these.

As an example on the processing of the signal from the probes, we have also shown on the graph the plot 7 representing the first filtered derivative of the signal, and illustrating the possibility of detection of end of purge by comparison of this signal with a predetermined threshold, exceeded by said derivative during its sudden growth 71 corresponding to the growth of the slope of the signal from the probe.

Likewise, there is also shown the plot 8 representing the second derivative, the zero crossing of which in decreasing threshold 81 can also be used as signaling information for end of purge.

The invention is not limited to the examples described above. It is more particularly intended for diesel engines but could also apply in general to all engines operating normally in lean mixture.

Claims

1. Method for managing the operation of a nitrogen oxide trap (18) for an internal combustion engine operating in a lean mixture, according to which a purge of said nitrogen oxide trap is periodically controlled, characterized in that 'there is a first oxygen sensor (21) on the exhaust pipe downstream of the nitrogen oxide trap (18), and we observe the evolution of a significant signal representative of the signal (3, 4) supplied by this probe, a significant increase (34, 44) of this significant signal from a first plateau (32, 42) of substantially constant level, obtained following a variation (31, 41) consecutive to a passage of the engine d an operation ^"lean mixture operation to rich mixture, being used as an indicator to order the end of the purge ^*.

2. Method according to claim 1, characterized in that a second oxygen sensor (22) placed upstream of the nitrogen oxide trap (18) is used in addition, to provide a reference signal (5) relative to to which the evolution of the signal supplied by the first probe is compared to provide said significant signal.

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

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

5. Method according to claim 1 or 2, characterized in that the increase in the significant signal is detected by making the difference between the instantaneous value of the significant signal and a sliding average of said signal, and by comparing this difference with a threshold.

6. Method according to claim 1 or 2, characterized in that the increase in the significant signal for a lambda probe is detected by comparing the voltage value delivered by the probe to a predetermined threshold.

7. Method according to claim 1, characterized in that the oxygen probe (s) (21, 22) are chosen from among the probes of the following type: lambda type probe, proportional oxygen probe, oxide sensor d 'nitrogen, of which ^' the oxygen concentration measurement function is used.

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

9. Device for managing the operation of a nitrogen oxide trap (18) for an internal combustion engine operating in a lean mixture, for implementing the method according to one of claims 1 to 7, the engine being equipped with an exhaust line (16) provided with a nitrogen oxide trap (18), characterized in that it comprises an oxygen sensor (21) placed on the exhaust line downstream of the trap with nitrogen oxide, and calculation means (23) for determining a significant increase in a significant signal representative of the signal (3, 4) supplied by said probe from a first plate (32, 42) of substantially constant level, obtained following the initiation of a purge operation, and use it as an indicator to control the end of the purge.

10. Device according to claim 9, characterized in that it comprises a second oxygen sensor (22) placed upstream of the nitrogen oxide trap (18) and connected to said calculation means (23) to provide them with a reference signal.