FR2943382A1 - Nitrogen oxide trap managing method for exhaust line of internal combustion engine of motor vehicle, involves determining quantity of sulfur in trap from measurement of carbon monoxide in gas at outlet of trap during regeneration of trap - Google Patents

Abstract

The method involves temporarily increasing richness and temperature of a gas mixture that traverses a nitrogen oxide trap (9), for regenerating the trap by purging nitrogen oxide. Quantity of sulfur contained in the trap is determined from measurement of carbon monoxide present in gas at an outlet of the trap during the regeneration of the trap. The sulfur quantity is determined based on difference between a value of carbon monoxide measured at the starting of the trap regeneration and a value measured at the end of the trap regeneration. An independent claim is also included for a nitrogen oxide trap device mounted in an exhaust line of an internal combustion engine of a motor vehicle, comprising a sensor.

Description

B08-0316EN - AxC / EVH

Société par Actions Simplifiée known as: RENAULT s.a.s. Management method of a nitrogen oxide trap and associated trap device Invention of: BATISSON Cathy GIRODON Alain

The present invention relates generally to the management of a trap for nitrogen oxides mounted in the exhaust line of an internal combustion engine of the invention. motor vehicle, and a nitrogen oxide trap device that can be used during the implementation of such a method. In order to reach the thresholds required for the emission of gaseous pollutants from motor vehicles, exhaust gas after-treatment systems disposed in the exhaust line of the engine are generally used. These systems make it possible in particular to reduce the emissions of nitrogen oxides, carbon monoxide and unburned hydrocarbons.

Unlike conventional oxidation catalyst devices mounted in the exhaust line of internal combustion engines, advantageous aftertreatment systems include trap devices operating discontinuously or alternatively. In normal engine operation, these devices trap pollutants and accumulate gradually without treating them. The treatment is done later, during regeneration phases. To be regenerated, these traps require engine specific modes of combustion, in order to guarantee temperature levels in the device and / or appropriate mixing resources for the removal of pollutants that have been accumulated. The temperature rise may, alternatively, be obtained by electric heating means or by fuel injection directly in the exhaust line upstream of the trap to be regenerated.

Among these trap devices, nitrogen oxide traps, which may be independent devices mounted in the exhaust line or zones dedicated to the accumulation of nitrogen oxides NOx provided in a catalyst device, are particularly known. conventional oxidation. Such nitrogen oxide traps thus make it possible to accumulate the nitrogen oxides produced by the combustion engine during its normal operation before being eliminated by reduction, which allows the conversion of the nitrogen oxides into nitrogen and nitrogen. carbon dioxide during regeneration or purge phases, which are triggered when a determined threshold is reached for the amount of nitrogen oxides accumulated. The trap therefore has periodic operation. The oil and fuel used in internal combustion engines contain small amounts of sulfur that are found in the exhaust gas as sulfur compounds. These sulfur compounds compete with the nitrogen oxides in the nitrogen oxide trap, thus progressively decreasing its ability to store nitrogen oxides. In other words, the efficiency of a NOx trap decreases with the accumulation of sulfur in the trap. To avoid excessive degradation of the operating characteristics of the nitrogen oxide trap, it is necessary to periodically remove the sulfur compounds during a so-called desulfurization step, using a rich mixture constituting a reducing medium, raised to a sufficient temperature in the nitrogen oxide trap. However, the desulfurization phases should preferably be limited in their duration and frequency, so as not to excessively reduce the lifetime of the nitrogen oxide trap or cause excessive fuel consumption. Indeed, each desulfurization operation has an impact on the mechanical strength of the engine and its exhaust line because of the high temperatures that are necessary from the engine outlet into the nitrogen oxide trap, to obtain a desulfurization suitable. In addition, during the desulfurization phases, there is a dilution of the fuel in the oil and an increase in fuel consumption. Finally, it is not possible to carry out a desulfurization operation in all the operating conditions of the heat engine.

For all these reasons, it is important to know precisely and accurately the amount of sulfur stored in a nitrogen oxide trap in order to trigger a desulfurization operation only wisely. This knowledge is useful not only in normal operation of the nitrogen oxide trap during which the nitrogen oxides and other constituents of the exhaust gases are retained in the traps, but also during the regeneration phases of the oxidation trap. nitrogen, by removing the nitrogen oxides stored and during the desulfurization phases by removal of sulfur compounds. Patent application WO 02/14666 (ROBERT BOSCH) discloses a process for desulfurizing a nitrogen oxide trap in which the mass of sulfur contained in the trap is detected using a signal coming from an oxygen sensor placed downstream of the nitrogen oxide trap. Such a method does not, however, make it possible to obtain a very precise determination of the quantities of sulfur stored in the nitrogen oxide trap. In addition, it requires the installation of a specific sensor in the form of an oxygen sensor. The patent application EP 0 962 639 (RENAULT) describes a method and a device for controlling the purge of sulfur oxides from a catalytic converter in which the quantity of sulfur stored in a nitrogen oxide trap is evaluated from average oil consumption. The amount of sulfur stored is proportional to the sulfur entering the trap, this proportion being given by mapping according to the operating points of the engine. This therefore assumes that the amount of sulfur emitted by the engine and the storage efficiency of the sulfur in the nitrogen oxide trap can be correctly modeled. However, the amount of sulfur emitted by a heat engine depends on the sulfur concentration of the fuel and the oil used, as well as the fuel and oil consumption. These quantities are poorly known, vary from one engine to another, and change over time. In addition, the storage capacity of sulfur compounds by a nitrogen oxide trap is not perfectly known. Since sulfur is not stored homogeneously in the nitrogen oxide trap, there is not always a good correlation between the amount of sulfur stored and the need for a desulphurization operation.

During a desulphurization operation, the modeling of the sulfur eliminated is carried out in an open loop. If the model underestimates the amount of sulfur remaining in the trap, the duration and frequency of the desulfurization phases are too low and the nitrogen oxides treatment capacity by the trap is progressively degraded. Conversely, if the model overestimates the amount of sulfur remaining in the nitrogen oxide trap, the desulphurisation phases are longer and more frequent than would be necessary, which translates into excessive consumption, decrease in the lifetime of the nitrogen oxide trap and an increase in the dilution of the oil in the fuel. The object of the present invention is to solve these various problems and to improve the accuracy of the determination of the amount of sulfur contained in a nitrogen oxide trap. The present invention also relates to a method of managing a nitrogen oxide trap by which the determination of the amount of sulfur accumulated is by simple and inexpensive means. Finally, the present invention also aims to facilitate the control of desulfurization operations of a nitrogen oxide trap and to control the duration continuously. According to one embodiment, a method for managing a nitrogen oxide trap mounted in the exhaust line of an internal combustion engine of a motor vehicle comprises, in particular, a stage of regeneration by purging the oxides of nitrogen. temporary wealth and temperature passes through the trap. During the setting determines the amount of sulfur contained in measuring carbon monoxide (CO) in them during the regeneration step. nitrogen by increasing the gaseous mixture which operates the process, it is trapped from a gas at the outlet of the trap I1 thus becomes possible to accurately determine the state of sulfur loading of the nitrogen oxide trap and to decide in a timely manner triggering one or more desulphurization operations of said trap.

The determination of the amount of sulfur contained in the trap is particularly effective according to this method because the carbon monoxide CO is, in the gas stream from the trap, the compound whose quantity is most influenced by the presence of the sulfur contained in the trap.

In addition, the use of a CO sensor, placed downstream of the nitrogen oxide trap, makes it possible to detect the end of the regeneration or purge stage of the trap by elimination of the nitrogen oxides. It is therefore not necessary to provide a second specific sensor for this purpose. In one embodiment, the determination of the amount of sulfur contained in the trap is made taking into account the difference between a value of CO measured at the beginning of the regeneration step by purging nitrogen oxides and a measured value after a specified period. The aforementioned fixed duration corresponds for example to the end of the regeneration step. In another embodiment of the method, the determination is made taking into account an integration value of the value of CO measured during the purge regeneration step of the nitrogen oxides.

It is thus possible to determine the need for desulfurization of the trap while controlling the progress of the desulfurization. It is also possible to take into account the quantity of nitrogen oxides contained in the trap at the beginning of the regeneration stage and / or a value derived from the CO value at the end of the regeneration stage.

The different criteria for determining the amount of sulfur contained in the trap can be implemented independently of each other or in combination.

It is thus possible to determine the need for desulfurization of the trap and to trigger a desulphurization step depending on the result of the determination of the amount of sulfur contained in the trap. It is also possible to control the desulfurization and to control its duration as a function of the result of the determination of the quantity of sulfur contained in the trap, this determination being made during the desulfurization stage. According to another advantageous embodiment, the process comprises a step of desulfurizing the nitrogen oxide trap followed by a step of purging the nitrogen oxides. During the purging step of the nitrogen oxides, the result of the determination of the amount of sulfur contained in the trap is used to check the efficiency of the desulphurisation step and to trigger, if necessary, a new one. desulfurization step.

In another aspect, it is further proposed a nitrogen oxide trap device for mounting in the exhaust line of a motor vehicle internal combustion engine. The device comprises a sensor capable of measuring the carbon monoxide (CO) present in the gas at the exit of the trap when the latter is in operation. The sensor may advantageously be placed immediately downstream of the trap or downstream of an area having a nitrogen oxide trap function provided in a catalyst device mounted in the exhaust line of an internal combustion engine.

The invention will be better understood from the study of an embodiment and implementation described by way of non-limiting example and illustrated by the accompanying drawings in which: - Figure 1 shows schematically the main elements of a internal combustion engine and its exhaust line; FIG. 2 shows the time course of the signal of a carbon oxide sensor located downstream of the nitrogen oxide trap during a regeneration and purge phase of the nitrogen oxides; and FIG. 3 shows the course as a function of time of the signal of the same sensor during a purge operation of the sulfur compounds by periodic modification of the composition and the temperature of the exhaust gases passing through the nitrogen oxide trap. FIG. 1 shows, by way of example, an internal combustion engine of supercharged type. The engine, referenced 1 as a whole, is associated with an exhaust line referenced 2 as a whole. The cylinders 3 of the engine 1 are fed by an intake manifold 4 into a mixture of compressed air and partially recycled exhaust gas. Fuel is injected into each of the cylinders 3 by injectors 3a connected to an injection manifold 3b. The exhaust gases from the engine are taken up by an exhaust manifold 5. Part of the exhaust gas is recycled by a bypass 6 having a valve 7 to control the amount of recycled exhaust gas. A turbocharger 8 comprises a compressor 8a which compresses the air at the intake of the engine. The compressor 8a is mechanically connected to a turbine 8b driven in rotation by the exhaust gases coming from the exhaust manifold 5. At the outlet of the turbine 8b, the exhaust gases first pass through a trap of oxides. nitrogen 9, then a device 10 for filtering soot particles.

In the example illustrated, a CO carbon monoxide measuring sensor 11 is placed in the exhaust line 2 immediately downstream of the nitrogen oxide trap 9. Alternatively, the nitrogen oxide trap could be constituted by an area dedicated to this function, provided inside a catalytic device. In this case, the sensor 11 would be placed downstream of the catalytic device. An electronic control unit UCE referenced 12 in the figure, receives, by connections not shown, different signals from the engine 1 as well as control signals from the driver of the vehicle. The electronic control unit 12 is also capable of transmitting control signals to the engine 1, for example by means of the connection 13 of the control signals of the fuel injection into the cylinders 3. The signal emitted by the The sensor 11 is transmitted to the electronic control unit 12 via the link 14. One of the aims of a process for managing the nitrogen oxide trap 9 is to allow an accurate determination of the desulfurization requirement of the oxide trap. of nitrogen, which makes it possible to control later stages of desulphurisation and to control their duration.

The desulphurization requirement of the nitrogen oxide trap 9 is determined from a measurement of the carbon monoxide present in the exhaust gas at the outlet of the nitrogen oxide trap 9 during a regeneration step, that is to say, in the illustrated example, a step during which the operating conditions of the heat engine are modified so as to increase the richness and the temperature of the gases passing through the nitrogen oxide trap 9 to eliminate the nitrogen oxides which have been accumulated in the trap 9. FIG. 2 illustrates in particular the appearance of the signal of the carbon monoxide sensor 11 during a purge phase of the nitrogen oxide trap 9. In FIG. , the abscissa, the time and ordinate, the signal of the sensor 11 which is proportional to the amount of CO measured. A purge phase of the nitrogen oxides starts at time t0. There is an increase in the amount of CO detected by the sensor in FIG. 2, this quantity remaining at first approximately constant. Then, a peak corresponding to a maximum of carbon monoxide at time t1 appears for an ideal operation without sulfur present in the nitrogen oxide trap 9 (dashed curve), and at time t2 for operation with presence of sulfur in the nitrogen oxide trap (solid curve). As can be seen, the solid line curve corresponding to the presence of sulfur shows a maximum of carbon monoxide less than the dashed curve which corresponds to a sulfur-free operation. Moreover, the time t1 is later than the time t2, the maximum of the dotted curve occurring after the maximum of the solid line curve. Analysis of the carbon monoxide sensor signal during NOx trap purging determines the amount of sulfur stored in the trap. For this purpose, several criteria can be used. For example, it is possible to use the difference between the values of the signal at the beginning of the purge operation and after a specified purge time or at the end of the purge operation.

It is also possible to integrate the carbon monoxide sensor signal during the duration of the purge. In a third implementation variant, the determination of the amount of sulfur stored in the trap can be made taking into account a value derived from the carbon monoxide value measured by the sensor at the end of the step of regeneration. When one or a combination of these criteria reaches a threshold determined by prior routine testing, it can be inferred that a desulfurization step has become necessary.

To achieve such desulphurization, consisting in purging the sulfur compounds that have been accumulated in the nitrogen oxide trap 9, it is advantageous to temporarily modify the operation of the heat engine 1 by increasing the richness of the mixture, that is to say the amount of fuel injected, which has the effect of increasing the temperature of the exhaust gas and therefore the temperature in the nitrogen oxide trap 9. A temperature close to 700 ° C can for example be reached. To avoid, however, that an excessive increase in the temperature in the nitrogen oxide trap results, which can lead to emissions of undesirable pollutants, it is preferable to carry out the desulphurisation step by splitting the operating phases at a high temperature. high and at high temperature. Between these different phases, operating phases of the engine are mixed lean and at lower temperature, periodically.

FIG. 3 illustrates the appearance of the signal of the carbon monoxide sensor 11 during such a fractional desulfurization step. On the abscissa, the time and the ordinate are represented by the signal of the sensor. The solid lines correspond to the presence of sulfur in the nitrogen oxide trap, while the dashed lines correspond to a thermal operation in the absence of sulfur in the nitrogen oxide trap. As can be noted, during each of the phases of operation in rich mixture, the signal of the carbon monoxide sensor increases rapidly to decrease during operation in lean mixture while falling to a value close to zero. The maximums reached during each of the phases of operation in a rich mixture gradually increase from the beginning of the indicated desulfurization phase t0 to a time noted t1 which corresponds to the end of the beginning of the desulfurization step. It is also noted in Figure 3 that the maximums of the measurements made by the carbon monoxide sensor are significantly higher when the nitrogen oxide trap does not include sulfur. In addition, when the amount of sulfur present in the nitrogen oxide trap is greater, the amounts of available CO downstream are lower. It is therefore possible to correlate the end of the desulfurization step with the achievement of a determined threshold of measured carbon monoxide. The curves obtained in FIG. 3 thus make it possible to verify the accuracy of a model of the sulfur stored in the nitrogen oxide trap, taking into account the value delivered by the sensor 11 during the phases of operation in a rich mixture of desulfurization. The control of the desulphurisation, and in particular the control of the duration of the phases of operation in rich mixture as well as their frequency, can be carried out according to the determination of the quantity of sulfur which was obtained before by the measurement of monoxide of carbon during a regeneration phase purge nitrogen oxides, as shown in Figure 2.

During a subsequent regeneration phase following a desulfurization step, it is possible, by means of the various criteria mentioned above, and considering the measurements made by the carbon monoxide sensor represented for example on a curve similar to that of Figure 2, determine the degree of sulfur poisoning of the nitrogen oxide trap. If it is confirmed that the previously carried out desulphurization step has been effective, the carbon monoxide measurement can be used to derive a sulfur poisoning criterion from the nitrogen oxide trap to recalibrate the model. sulfur stored. It is also possible to continue the normal operation of the internal combustion engine with the different successive phases of regeneration of the nitrogen oxide trap. If on the contrary, there is again a too large amount of sulfur, corresponding to a poisoning exceeding a predetermined threshold, can be triggered a new desulfurization step. If after a series of desulfurization steps followed by regeneration steps by removal of the nitrogen oxides, the amount of sulfur always remains above a determined threshold, it is then possible to deduce that the oxide trap of nitrogen is defective or too old, so it is necessary to replace it. The present invention thus makes it possible to obtain an accurate and reliable result on the quantity of sulfur compounds capable of poisoning a nitrogen oxide trap. The invention makes it possible to trigger desulfurization steps conveniently. It is also possible to precisely control the desulfurization in successive phases. In addition, the use of such a carbon monoxide sensor makes it possible both to determine the amount of sulfur contained in the nitrogen oxide trap and to detect the end of a purge step of the nitrogen oxides. corresponding to a regeneration of the trap. The use of a single sensor simplifies the device without affecting the accuracy of the management of the nitrogen oxide trap.

Claims (10)

REVENDICATIONS1. Process for the management of a nitrogen oxide trap (9) mounted in the exhaust line (2) of an internal combustion engine (1) of a motor vehicle, comprising in particular a step of regeneration by purging the oxides of nitrogen nitrogen by temporarily increasing the richness and temperature of the gaseous mixture passing through the trap, characterized in that the amount of sulfur contained in the trap (9) is determined from a measurement of carbon monoxide ( CO) in the gases at the exit of the trap during the regeneration step. 2. The method of claim 1 wherein the determination is made taking into account the difference between a value of CO measured at the beginning of the purge regeneration step nitrogen oxides and a measured value after a fixed period. 3. Process according to claim 2, in which the determined duration corresponds to the end of the purge regeneration step of the nitrogen oxides. 4. Method according to one of the preceding claims, wherein the determination is made taking into account an integration value of the value of CO measured during the purge regeneration step nitrogen oxides. 5. The method of claim 4 wherein it also takes into account the amount of nitrogen oxides contained in the trap at the beginning of the regeneration step. 6. Method according to one of the preceding claims, wherein the determination is made taking into account a value derived from the value of CO at the end of the regeneration step. 7. Method according to one of the preceding claims comprising a nitrogen oxide trap desulphurization step, in which the desulfurization is triggered depending on the result of the determination of the amount of sulfur contained in the trap. 8. Method according to one of the preceding claims comprising a step of desulphurization of the nitrogen oxide trap, wherein the desulfurization is piloted and its duration is controlled as a result of a determination of the amount of sulfur contained in the trap. , made during the desulfurization step. 9. Method according to one of the preceding claims comprising a step of desulphurization of the nitrogen oxide trap followed by a purge step of the nitrogen oxides, wherein, in the step of regeneration by purging the oxides of nitrogen. nitrogen, the result of the determination of the amount of sulfur contained in the trap is used to check the efficiency of said desulfurization step and trigger if necessary a new desulfurization step. Nitrogen oxide trapping device intended to be mounted in the exhaust line (2) of an internal combustion engine (1) of a motor vehicle, characterized in that it comprises a sensor (11) capable of measuring the carbon monoxide (CO) present in the gases at the exit of the trap during a regeneration step of said trap.