FR2921970A1 - Exhaust line for heat engine of motor vehicle, has catalytic purification unit for purifying exhaust gas crossing bypass pipe, where unit has nitrogen oxide trap intercalated in downstream section between valve and gas releasing outlet - Google Patents

Abstract

The line (10) has a main exhaust gas pipe (20) comprising a nitrogen oxide trap (26) intercalated between upstream and downstream sections (22, 24). A bypass pipe (28) is extended from a T-shaped intersection (30) with the upstream section until another T-shaped intersection (32) e.g. three way valve, with the downstream section through bypassing of the trap. A catalytic purification unit purifies the exhaust gas crossing the bypass pipe. The unit has another nitrogen oxide trap (38) intercalated in the downstream section between the valve and an exhaust gas releasing outlet (14).

Description

BACKGROUND OF THE INVENTION The present invention generally relates to motor vehicle exhaust lines equipped with nitrogen oxide traps.

More specifically, the invention relates to an automobile vehicle exhaust line, of the type comprising: a main exhaust gas flow duct, comprising an upstream section, a downstream section, and an oxide trap; nitrogen interposed in series between the upstream and downstream sections; a bypass duct extending from an intersection with the upstream section to an intersection with the downstream section by passing the nitrogen oxide trap. Such an exhaust system is known from FR-2 886 337. Decreasing the discharge of pollutants into the atmosphere is a constant concern of car manufacturers, in order to comply with the standards, which are increasingly severe, and protect the environment. In this context, the invention aims to provide a more efficient exhaust line in depollution and to reduce the consumption of the vehicle.

To this end, the invention relates to an exhaust line of the aforementioned type, characterized in that it comprises means for catalytic purification of the exhaust gas passing through the bypass duct. Advantageously, the means for catalytic purification of the exhaust gas passing through the bypass ducts may comprise a second nitrogen oxide trap interposed in series in the downstream duct downstream of the intersection between the bypass duct and the downstream duct. In addition, the two traps nitrogen oxides can be separated, according to the course of the exhaust line, a distance of between 0.5 meters and 1.5 meters.

Alternatively, the means of catalytic purification of the exhaust gas passing through the bypass duct may comprise a three-way catalyst interposed in series in the downstream duct, downstream of the intersection between the bypass duct and the downstream duct.

According to another embodiment of the invention, the means of catalytic purification of the exhaust gas passing through the bypass duct may comprise a three-way catalyst interposed in series in the bypass duct.

The exhaust line may also include means for directing the exhaust gas from the upstream section selectively to either the bypass line or the nitrogen oxide trap. Other features and advantages of the invention will emerge from the detailed description given below, by way of indication and in no way limiting, with reference to the appended figures, in which: FIG. 1 schematically represents a line of FIG. exhaust according to a first embodiment of the invention; - Figure 2 is a view similar to that of Figure 1, and showing a second embodiment of the invention; and - Figure 3 is a view similar to those of Figures 1 and 2, and illustrating a variant of the second embodiment. The exhaust line 10 illustrated in Figure 1 is intended to be connected to the output of a thermal engine operating normally in lean mixture.

The exhaust line thus has an inlet 12 adapted to be connected to the exhaust cylinder of the engine and an outlet 14 for releasing the exhaust gases into the atmosphere, after purification. The inlet 12 is formed of a manifold 16 adapted to capture the exhaust gases of the engine. The manifold 16 includes a plurality of inlets 17 each connectable to the outlet of a combustion chamber of the engine. The outlet of the collector 16 is connected to the inlet of a catalytic purification unit 18 known per se, for example a three-way catalyst. Downstream of the catalyst 18, the line comprises a main circulating pipe 20, the end of which constitutes the outlet 14 for releasing the exhaust gas into the atmosphere. The main conduit 20 comprises an upstream section 22, a downstream section 24 and a first nitrogen oxide trap 26 interposed in series between the upstream and downstream sections 24. The notions of upstream and downstream are here understood relative to the flow direction of the exhaust gas. Line 10 also includes a bypass duct 28 extending from a T-intersection 30 to another T-intersection 32 by passing the first nitrogen oxide trap 26. At the intersection T 30, the bypass duct 28 is connected to the upstream section 22 of the main duct. At the T-intersection 32, the bypass duct 28 is connected to the downstream section 24 of the main duct. The T-intersection 32 is a three-way valve controlled by an actuator 34, itself controlled by a computer 36. The three-way valve comprises a flap (not shown) movable between a first position in which the bypass section 28 is isolated, and a second position in which the first nitrogen oxide trap 26 is isolated. In the first position of the valve, the exhaust gases are able to circulate from the upstream section 22 through the nitrogen oxide trap 26 to the three-way valve 32 and then through the downstream section 24 to the the outlet 14. In the first position, the gases do not circulate in the bypass duct 28. In the second position, the exhaust gases are able to flow from the upstream section 22 through the bypass section 28 until at the three-way valve 32 and then through a portion of the downstream section 24 to the outlet 14. In the second position, the exhaust gas does not pass through the nitrogen oxide trap 26. In a first embodiment 1, the exhaust line further comprises a second nitrogen oxide trap 38, interposed in the downstream section between the three-way valve 32 and the outlet 14. Thus, that the valve of the valve three tracks either in his first or in his second positi The exhaust gases still pass through the second nitrogen oxide trap before being released into the atmosphere via the outlet 14. The exhaust line described above has multiple advantages. Because it comprises a second nitrogen oxide trap interposed in series in the downstream duct, downstream of the intersection of the bypass duct and the downstream duct, the exhaust gases still pass through the second oxide trap nitrogen, irrespective of the position of the valve of the three-way valve. In addition, the second nitrogen oxide trap is located at a distance from the first nitrogen oxide trap. The exhaust gases travel between the two nitrogen oxide traps, a distance of between 50 cm and 1 m. As a result, the temperature of the exhaust gas at the second nitrogen oxide trap is significantly lower than at the first nitrogen oxide trap. The temperature of the exhaust gas at the second nitrogen oxide trap is 50 ° C to 100 ° C lower than the temperature of these same exhaust gases at the first nitrogen oxide trap. This has several advantages. Because the second nitrogen oxide trap is exposed to lower temperature gases, its aging will be slower.

In addition, the nitrogen oxide traps are provided to operate between substantially 250 ° C and 450 ° C. Outside this temperature zone, the efficiency of the nitrogen oxide trap decreases sharply. When the engine is operating at a speed such that the exhaust gas reaching the first nitrogen oxide trap is at a temperature above 450 ° C, the second nitrogen oxide trap can take over from the first trap with nitrogen oxides. In this case, the computer 36 commands the three-way valve to direct the exhaust gases through the bypass circuit and to isolate the first nitrogen oxide trap. The exhaust gases arriving at the second nitrogen oxide trap will be in a temperature range corresponding to the maximum efficiency of this second nitrogen oxide trap. Thus, the exhaust line makes it possible to obtain a reduction in discharges, in particular of nitrogen oxides, in the case where the engine is operating at a high load. This makes it possible to widen the range of operation of the engine in a lean mixture and, consequently, to reduce the consumption of the vehicle.

Furthermore, the fact of having two nitrogen oxide traps in series makes it possible to increase the overall efficiency of the trapping, that is to say makes it possible to increase the proportion of pollutants, in particular nitrogen oxides, contained in the exhaust gas leaving the engine that is trapped in the exhaust line. Indeed, two traps in series, having respective volumes of determined sum and working one at a first temperature, and the other at a second temperature below 50 ° C to 100 ° C at the first temperature, are more effective a single volume trap equal to said determined sum and working at the first temperature. This is due in particular to the fact that the most downstream trap ages more slowly and retains better long-term efficiency. Therefore, for a targeted efficiency of the nitrogen oxide decontamination system, it is possible to use nitrogen oxide traps of smaller volume than in the state of the art. Thus, if, to obtain a targeted efficiency of the pollution control system, it is necessary to use a nitrogen oxide trap of volume determined in the state of the art, in the invention, the cumulative volume of the two traps nitrogen oxide will be between 70% and 90% of said determined volume, and will be worth for example 80% of said determined volume.

Moreover, if the valve in its first position is not leakproof and leaks, the exhaust gas passing the bypass 28 due to leakage will still cross the second nitrogen oxide trap before being rejected at the atmosphere. The impact of leaks at the three-way valve on the total emissions of pollutants in the exhaust line is therefore considerably reduced. Therefore, it becomes possible to use on the exhaust line valves having a lower level of tightness in the closed position of the bypass duct. These valves are less expensive than three-way valves with a high level of sealing used in the state of the art.

A second embodiment of the invention will now be described with reference to FIG. 2. Only the points by which the second embodiment differs from the first will be described below. The identical elements or providing the same functions will be designated by the same references as in the first embodiment. In the second embodiment, the exhaust line 10 does not comprise a second nitrogen oxide trap interposed in the downstream section between the three-way valve 32 and the outlet 14 for releasing the exhaust gas in the exhaust zone. 'atmosphere. On the other hand, the exhaust line comprises a three-way catalyst 40 interposed in series in the bypass line 28 between the T-intersection 30 and the three-way valve 32. The three-way catalyst 40 is of the same type as the catalyst 18.

As known per se, it is able to reduce NOx to N2 gas, and to oxidize CO and HC to CO2. Such a catalyst makes it possible to reduce NOx in N2 when the engine of the motor vehicle operates in a rich or stoichiometric mixture, but not in a lean mixture. The mixture is said to be stoichiometric when, in the combustion chambers, the amount of oxygen and the amount of fuel are in stoichiometric proportions. The mixture is said to be rich when an excess fuel is introduced into the combustion chambers in comparison with the stoichiometric conditions. The mixture is said to be poor when excess air is introduced into the combustion chambers with respect to the stoichiometric conditions. The engine operates under stoichiometric conditions or in rich mixture when the vehicle is traveling at high speed, or at high load, for example on a hill. The exhaust gases then reach high temperatures. On the contrary, the engine runs lean in other cases, the exhaust gas then being colder. The three-way catalysts have the advantage of being able to operate at high temperatures, while the nitrogen oxide traps such as 26 contain a substrate which is destroyed at temperatures above 800 ° C.

A variant of the second embodiment is shown in FIG. 3. In this variant, the three-way catalyst 40 is not placed in the bypass duct. In contrast, the three-way catalyst 40 is interposed in series in the downstream section 24, downstream of the intersection 32 between the bypass duct 38 and the downstream section 24.

The exhaust line of FIG. 2 presents a large part of the advantages of the first embodiment. The three-way catalyst 40 takes over from the nitrogen oxide trap 26 when the exhaust gas exceeds a predetermined maximum temperature, for example 450 ° C. The computer 36 controls, in this case, the three-way valve to direct the exhaust gas to the bypass circuit. The nitrogen oxide trap 26 is thus bypassed when the vehicle is traveling at high speed or at high load, that is to say in cases where the engine is operating under stoichiometric or rich conditions. Under these conditions, the three-way catalyst arranged in the bypass line or in the downstream section downstream of the valve 32 is able to convert NOx to N2. The three-way catalyst interposed in the bypass line traps pollutants passing through the bypass line in the event of leakage of the three-way valve. Total emissions of pollutants from the exhaust line are therefore reduced. It is possible to use a three-way valve having a lower level of sealing but which is less expensive. In the case where the three-way catalyst is disposed downstream of the three-way valve 32, the fact of having a nitrogen oxide trap and a three-way catalyst in series makes it possible to increase the proportion of pollutants that are trapped. When the engine is operating in a lean mixture and the NOx trap 26 is not bypassed, the HCs and COs undergo two decontamination stages, namely the NOx trap 26 and the three-way catalyst 40. NOx, on the other hand, pass through only one decontamination stage, the NOx trap 26.

The second embodiment therefore also allows to widen the operating range of the engine in which the exhaust gases undergo a depollution, especially when the NOx trap 26 is bypassed. Thus, whatever the embodiment of the invention, the exhaust line comprises means for catalytic purification of the exhaust gas passing through the bypass duct. This has the effect of widening the operating range of the engine in which NOx decontamination is effective, whether during normal engine cycles or off-cycle. This also makes it possible to reduce the discharges of other pollutants, for example HC or CO. This finally makes it possible to optimize the performance of the three-way valves, that is to say to choose a three-way valve a little less good, without compromising the performance of the exhaust line in terms of pollutant discharge in the air.

The first embodiment is more expensive than the second because a NOx trap is more expensive than a three-way catalyst. On the other hand, it makes it possible to operate the engine in poor on a wider range of operating points, without increasing the rejections. It also makes it possible to depollute the exhaust gases even when the engine is operating in a stoichiometric or rich mixture, and that the NOx trap 26 is bypassed. The second embodiment is more economical, but does not expand the range of operating points in which the engine runs lean. It is necessary for the engine to work in a rich or stoichiometric mixture so that the three-way catalyst traps the NOx when the NOx trap 26 is bypassed. The exhaust line described above can have multiple variants. The three-way valve 32 may not be disposed at the intersection of the bypass duct and the downstream section but rather at the intersection 30 between the upstream duct and the bypass section. The exhaust line may also not include a three-way valve, but rather two simple valves, one likely to close the bypass duct 28 and the other likely to prohibit the flow of exhaust gas through the first trap with nitrogen oxides.

Claims (6)

1.- Exhaust line of a motor vehicle, the exhaust line (10) comprising: - a main duct (20) for the circulation of exhaust gases, comprising an upstream section (22), a downstream section ( 24), and a nitrogen oxide trap (26) interposed in series between the upstream and downstream sections (22, 24); a bypass duct (28) extending from an intersection (30) with the upstream section (22) to an intersection (30) with the downstream section (24) by bypassing the oxide trap; nitrogen (26), characterized in that it comprises means (38, 40) for catalytic purification of the exhaust gases passing through the bypass duct (28). 2. Exhaust line according to claim 1, characterized in that the means for catalytic purification of the exhaust gas passing through the bypass duct (28) comprises a second trap nitrogen oxides (38) interposed in series in the duct downstream (24), downstream of the intersection (30) between the bypass duct (28) and the downstream duct (24). 3. Exhaust line according to claim 2, characterized in that the two nitrogen oxide traps (26; 38) are separated, according to the course of the exhaust line (10), a distance between 0.5 meters and 1.5 meters. 4. Exhaust line according to claim 1, characterized in that the means for catalytic purification of the exhaust gas passing through the bypass duct (28) comprises a three-way catalyst (40) interposed in series in the downstream duct (24). downstream of the intersection (30) between the bypass duct (28) and the downstream duct (24). 5. Exhaust line according to claim 1, characterized in that the catalytic purification means of the exhaust gas passing through the bypass duct (28) comprises a three-way catalyst (40) interposed in series in the bypass duct (28). ). 6. Exhaust line according to any one of claims 1 to 5, characterized in that it comprises means (32, 34, 36) for directing the exhaust gas from the upstream section (22) selectively be to the bypass line (28) or to the nitrogen oxide trap (26).