FR2921969A1 - Exhaust gas post-treating device for oil combustion engine of motor vehicle, has secondary line extended from one part and joined to another part of main line such that valve orients gas flow into secondary line to prevent filter racing - Google Patents

Abstract

The device has a particle filter (3) placed downstream of a nitrogen oxide trap (1), and another nitrogen oxide trap (2) placed downstream of the filter. A secondary exhaust line (11) is extended from a part of a main exhaust line (10) and joined to another part of the line (10) such that a direction changing valve (6) orients an exhaust gas flow into the line (11) for preventing racing of the filter. The former part is located upstream of the filter and between the trap (1) and the filter. The latter part is located downstream of the filter and between the filter and the trap (2). An independent claim is also included for a control method for implementing a post-treating device.

Description

EXHAUST GAS POST-TREATMENT DEVICE ARRANGED IN AN EXHAUST LINE FOR A DIESEL COMBUSTION ENGINE

The invention relates to the field of diesel depollution, and more particularly relates to a device for aftertreatment of exhaust gases. To meet the lower thresholds for emissions of gaseous pollutants from motor vehicles, increasingly complex after-treatment gas devices are arranged in the exhaust line of lean-burn engines. These devices make it possible in particular to reduce particulate and nitrogen oxide emissions in addition to carbon monoxide and unburned hydrocarbons. Unlike a traditional oxidation catalyst, these devices operate discontinuously or alternatively, that is, in normal operation they trap the pollutants but treat them only during the purge phases. Thus to be regenerated, these traps require specific modes of combustion to ensure the necessary temperature and / or wealth levels. A nitrogen oxide trap or NOxTrap is used to treat the nitrogen oxides produced by diesel combustion in the engine cylinders and exhausted. The NOxTrap is a post-treatment system that has a discontinuous or alternating operation, alternating storage phases where the nitrogen oxides or NOx present in the exhaust gases from the engine are retained in the NOxTrap by chemical reaction, and purge phases during which the NOx stored in the trap are reduced, that is to say removed from the trap. The storage phase occurs when the exhaust gas is in a poor environment (excess of oxygen) while the purge phase takes place in the presence of a rich medium (excess fuel). Thus, in order for a nitrogen oxide trap to function properly and to limit NOx emissions, it is necessary to be able to alternately ensure a operation with a richness greater than 1 in order to reduce the amount of nitrogen oxides trapped during the operation of the engine in lean mixture. However, a nitrogen oxides trap or NoxTrap stores the sulfur contained in the exhaust gas and is thus poisoned, which limits its storage efficiency of nitrogen oxides. Thus, it is necessary to be able alternately ensure a purge operating at greater than 1 richness bringing the trap at temperatures of substantially 600 ° C to treat the trapped sulfur. In an exhaust line architecture composed of a first nitrogen oxide trap, also called NOxTrap, or an oxidation catalyst (DOC) then a particle filter (FAP) and finally a second nitrogen oxide trap, the purge of the first nitrogen oxide trap is characterized by a rich phase which is maintained until the end of NOxTrap purge disposed downstream of the particulate filter. To ensure a rich operation greater than 1, the amount of reducing species relative to the amount of oxidizing species must be increased in the exhaust line upstream of the trap. Indeed, in this mode of operation the amount of oxygen in the exhaust line is low, unlike the amount of oxygen poor mixtures with normal injections. A technique conventionally used to be able to ensure such operation with a richness greater than 1, consists on the one hand in regulating the flow of air admitted and, on the other hand, in performing a post injection, that is to say in injecting. an additional amount of fuel in the engine cylinders at the end of the cycle. During these purging phases, the medium is rich and the reducing agents generate exothermic reactions, which increases the temperatures of the nitrogen oxide trap and the particulate filter. Under these conditions of high temperatures, the particulate filter loaded with both soot and unburned hydrocarbons (HC), has a high sensitivity to runaway, that is to say an uncontrolled combustion of particles, even for a low level of oxygen in the gases. In addition, during purge phases of the nitrogen oxide trap placed downstream of the particulate filter, the oxygen present in the exhaust gas is limited but when the purge phase is interrupted, for example in the deceleration phase or when returning to a normal injection, the oxygen arrives massively in the exhaust line and may cause runaway particle filter, and deteriorate. To prevent runaway of the particulate filter, the document FR2873164 proposes a device and a method which make it possible to increase the flow rate of the exhaust gas controlled by a quantity of air admitted into the cylinders when a runaway is sensed when the temperatures become high. This process allows evacuation of the heat resulting from the combustion of soot.

The document FR2860837 proposes an exhaust gas purification system which makes it possible to stop the regeneration when the measurement of the quantity of heat released in the particulate filter exceeds a threshold value or to reduce the quantity of oxygen arriving in the filter. particles by an increase in fuel injections when this amount of heat is below a threshold when switching to engine idling. An object of the present invention is to provide an improved alternative solution, simple and inexpensive, which avoids the runaway particle filter.

For this purpose, the subject of the invention is an exhaust gas after-treatment device disposed in a main exhaust line for a diesel combustion engine comprising a first oxidation means, a particulate filter, arranged in a downstream of the first oxidation means relative to the direction of the gas flow, a second oxidation means, disposed downstream of the particle filter relative to the direction of the gas flow, a secondary exhaust line connected bypass on the main exhaust line, a change of direction valve of the exhaust gas flow, characterized in that the secondary exhaust line originates in a part of the main exhaust line upstream of the particulate filter, between the first oxidizing means and the particulate filter, and discharging into a portion of the main exhaust line downstream of said particulate filter, between the filter particles and the second oxidation means so that the valve directs the flow of exhaust gas in the secondary exhaust line to prevent a runaway of the particulate filter.

Such an aftertreatment device of the exhaust gas makes it possible to prevent the particle filter from racing and limits its aging or its destruction.

According to other characteristics, the first and second oxidation means may be nitrogen oxide traps, the first oxidation means may be an oxidation catalyst and the second oxidation means may be an oxide trap. nitrogen. the valve can direct the exhaust gases in the secondary exhaust line during the purging phases of the second oxidation means when the temperature of the particulate filter is substantially greater than a predetermined threshold, the valve can direct the gases of exhaust in the secondary exhaust line during the purging phases of the second oxidation means when the amount of unburned hydrocarbons is substantially greater than a predetermined threshold, the change of direction valve of the exhaust gas flow can directing the flow of the exhaust gas into the secondary exhaust line during interruptions of the purge phases of the second oxidizing means when the oxygen level in the main exhaust line is substantially greater than a predetermined threshold, when the gases flow into the secondary exhaust line, the valve can direct the exhaust gases into the exhaust line main exhaust if the particle filter temperature is below a predetermined threshold and if the amount of unburned hydrocarbons is below a predetermined threshold, when the gases flow in the secondary exhaust line, the valve can direct the gases exhaust in the main exhaust line if the amount of oxygen in the main exhaust line is less than a predetermined threshold.

The invention also relates to a method for the implementation of such a device characterized in that directs the exhaust gas in the secondary exhaust line before the runaway of the particulate filter. According to other characteristics, the exhaust gases can be oriented in the secondary exhaust line o during the purging phases of the second oxidation means when the temperature of the particulate filter is substantially greater than the predetermined threshold, purge phases of the second oxidation means when the amount of unburned hydrocarbons is substantially greater than a predetermined threshold, o during interruptions of the purge phases of the second oxidation means when the oxygen level in the main line of exhaust is substantially greater than a predetermined threshold, when the gases flow in the secondary exhaust line, the exhaust gas can be oriented in the main exhaust line o if the temperature of the particle filter is below a threshold predetermined and if the amount of unburned hydrocarbons is less than a predetermined threshold, o if the amount of dioxyg in the main exhaust line is below a predetermined threshold.

The invention also relates to a motor vehicle comprising such a device for aftertreatment of the exhaust gases.

Other features, objects and advantages of the present invention will appear on reading the detailed description which follows, and with reference to the appended drawing, given by way of non-limiting example and in which: FIG. schematic assembly of the device according to the invention.

FIG. 1 shows an exhaust gas after-treatment device disposed on a duct or an exhaust line 10, downstream of a combustion engine 4, for example four-cylinder engine equipped with a injection system 5.

The exhaust line 10 is a main line of the flow of the exhaust gas, and comprises oxidation means 1, 2, 3. These means 1,2,3 oxidation are formed by a first trap of a nitrogen oxide called NoxTrap 1 or a diesel engine oxidation catalyst called DOC 1, followed by a particulate filter 3, downstream of which is a second nitrogen oxide trap 2, called NoxTrap 2. The main exhaust line 10 has a secondary line 11 for the flow of exhaust gas branched on the main line 10. The secondary line 11, or branch line, originates in a part of the exhaust line main located upstream of the particulate filter 3, between the first nitrogen oxide trap 1 and the particulate filter 3, and opens into a part of the main exhaust line 10 located downstream of the particulate filter 3, between the particulate filter 3 and the second nitrogen oxide trap 2. L a main exhaust line 10 comprises a valve 6 adapted to change the direction of the flow of exhaust gas, called the direction change valve of the exhaust gas flow, the valve 6 is disposed downstream of the particle filter 3, and allows the exhaust gas to flow either in the direction of the main exhaust line 10, NoxTrap 1 to the particulate filter 3, or in the direction of the secondary line 11, the NoxTrap 1 directly to the NoxTrap 2 without passing through the particulate filter 3. The changeover valve 6 directs the exhaust gases into the main exhaust line 10 when the engine 4 is operating in lean mixture to trap the soot in the particulate filter 3.

The change-of-direction valve of the exhaust gas flow can be urged to pass the flow of the exhaust gas into the bypass line 11 when the medium is at a richness greater than 1, during the phases in particular, the exhaust gas is circulated through the bypass line 11 when the temperature of the exhaust gas is very high, that is to say when the temperature measured by a not shown temperature probe disposed in the exhaust line is substantially greater than 500 ° C (case a). Likewise, the valve 6 is commanded to pass the exhaust gas into the bypass line 11 when the quantity of HC unburned hydrocarbons passing through the particle filter 3 is very large or substantially greater than a predetermined threshold; that is, when the amount of unburned hydrocarbons reaches values substantially greater than 1000 ppm (parts per million). The amount of unburned hydrocarbons is, for example, determined by modeling methods, making it possible to evaluate the level of unburned hydrocarbons as a function of the temperature of the exhaust gas as measured by the temperature probe 9 (case b). In another embodiment, the amount of unburned hydrocarbons is delivered by an HC sensor disposed upstream of the particulate filter 3 to directly measure the amount of unburned hydrocarbons passing through the particulate filter 3.

When the engine operates in a medium with a richness substantially greater than 1, the oxygen present in the exhaust gas is limited. However, if the purge phase of the second nitrogen oxide trap 2 is interrupted, for example during a deceleration phase or when returning to a normal injection, the oxygen can arrive in mass in the exhaust line. When this oxygen content is substantially greater than 2%, the particulate filter 3 can then become damaged. An oxygen sensor (not shown) disposed at the outlet of the combustion engine 4 makes it possible to detect a return of the oxygen in the main exhaust line 10. If the presence of oxygen is detected in the exhaust line, that is that is, if the oxygen content present in the exhaust line is substantially greater than a predetermined threshold, for example substantially equal to 2%, the valve 6 is actuated to pass the exhaust gases through the bypass line 11 (case c). When the exhaust gas flows in the secondary exhaust line 11, they can recirculate in the main line only if the risk of runaway is raised, for example when the temperature of the particulate filter is below a threshold predetermined and whether the unburned hydrocarbons are partly discharged, or if the amount of oxygen is substantially less than a predetermined threshold. Thus the valve 6 redirects the flow in the main exhaust line when the temperature of the particulate filter 3 is substantially less than 500 ° C and the amount of unburned hydrocarbons is substantially less than 1000 ppm. Such a device and such a method of controlling the changeover valve 6 allow the exhaust gas to flow in the bypass line 11 when the engine 4 operates in a rich mixture and when there is no risk of runaway. Thus, when none of the conditions a, b, c are satisfied, the exhaust line used for the flow of the exhaust gases is the main exhaust line 10. In doing so, the exhaust gases do not pass through. The particulate filter is not subject to uncontrolled combustion, and the particulate filter 3 does not pack. Such a device and such a control method make it possible to avoid premature deterioration of the particulate filter 3.

2. 25 3. 30 4.

Claims (10)

An exhaust aftertreatment device disposed in a main exhaust line (10) for a diesel combustion engine (4) comprising a. a first oxidation means (1) b. a particulate filter (3) disposed downstream of the first oxidizing means relative to the direction of gas flow, c. a second oxidation means (2) disposed downstream of the particle filter relative to the direction of gas flow, d. a secondary exhaust line (11) branched to the main exhaust line (10) e. a changeover valve (6) of the exhaust gas flow, characterized in that the secondary exhaust line (11) originates in a part of the upstream main exhaust line (10) of the particulate filter (3), between the first oxidizing means (1) and the particulate filter (3), and opening into a part of the main exhaust line (10) downstream of said particulate filter ( 3), between the particulate filter (3) and the second oxidizing means (2) so that the valve 6 orients the flow of the exhaust gas into the secondary exhaust line (11) to prevent runaway particle filter (3). 2. Exhaust aftertreatment device according to claim 1 characterized in that the first and second oxidation means are nitrogen oxide traps. 3. Exhaust aftertreatment device according to claim 1, characterized in that the first oxidation means (1) is an oxidation catalyst and the second oxidation means (2) is an oxide trap. nitrogen. 4. Exhaust aftertreatment device according to one of the preceding claims characterized in that the valve (6) orients the exhaust gas in the secondary exhaust line during the purge phases of the second means of oxidation (2) when the temperature of the particulate filter (3) is substantially greater than a predetermined threshold. 5. Aftertreatment device of the exhaust gas according to one of the preceding claims characterized in that the valve (6) orients the exhaust gas in the secondary exhaust line during the purge phases of the second means of oxidation (2) when the amount of unburned hydrocarbons is substantially greater than a predetermined threshold. 6. Aftertreatment device of the exhaust gas according to one of claims 1 to 4 characterized in that the valve (6) of change of direction of the flow of the exhaust gas orients the flow of the exhaust gases. exhaust in the secondary exhaust line (11) during interruptions of the purge phases of the second oxidation means (2) when the oxygen level in the main exhaust line (10) is substantially greater than a predetermined threshold . Exhaust aftertreatment device according to Claim 5, characterized in that when the gases flow in the secondary exhaust line (11), the valve (6) directs the exhaust gases into the main exhaust line (10) if the temperature of the particle filter (3) is below a predetermined threshold and the amount of unburned hydrocarbons is below a predetermined threshold. 8. Aftertreatment device of the exhaust gas according to claim 6 characterized in that when the gases flow in the secondary exhaust line (11), the valve (6) orients the exhaust gas in the main exhaust line (10) if the amount of oxygen in the main exhaust line is less than a predetermined threshold. 9. Control method for the implementation of a post-exhaust gas treatment device according to one of claims 1 to 8, characterized in that the flow of the exhaust gas is oriented in the line d secondary exhaust (11) before the particle filter (3) rushes. 10. A motor vehicle comprising an aftertreatment device for exhaust gas according to one of claims 1 to 8.