KR20070021266A - METHODS OF REGENERATING A NOx ABSORBENT - Google Patents

Abstract

The exhaust system 40 for the lean burn internal combustion engine 12 comprises at least one NO _x -absorbent disposed on a single monolithic carrier 42; Means (20) comprising an injector for injecting liquid reducing agent droplets into the exhaust gas upstream of the at least one carrier; And in operation, NO _X - means for controlling the injection of reducing agent in order to satisfy the emission standards related to playing the absorbent; includes, the liquid reducing agent droplets NO _X - into contact with the absorbent reduction of the localized NO _X It is configured to generate.

Exhaust system, monolith, carrier, exhaust gas, reducing agent, absorbent, catalyst

Description

METHODS OF REGENERATING A NOx ABSORBENT}

The present invention relates to an exhaust system for a lean burn internal combustion engine such as a diesel engine containing an absorbent for nitrogen oxides (NO _X ). In particular, the present invention relates to a method for regenerating a NO _x -absorbent.

A vehicle for absorbing nitrogen oxides (NO _X ) from lean exhaust gases, releasing stored NO _X into the atmosphere containing less oxygen and reducing them to nitrogen molecules (N ₂ ). This is known from EP 0560991 and the like. In general, NO _X -absorbents are associated with a catalyst for oxidizing nitrogen monoxide (NO) to nitrogen dioxide (NO ₂ ), such as platinum (Pt), and optionally NO _x to N ₂ with an appropriate reducing agent such as hydrocarbons. It is also associated with a catalyst such as rhodium for reduction. The NO _X-catalyst and the NO oxidation catalyst and the NO _X selective reducing catalyst comprising the NO _X absorbent is lean - also known as the trap-trap (lean NO _X -trap), or simply the NO _X.

In general NO _X -trap formulations, the NO _X -absorbents are compounds of alkali metals such as potassium and / or cesium; Alkaline earth metal compounds such as barium or strontium; And / or compounds of rare earth metals, such as lanthanum and / or yttrium in general. According to the mechanism generally provided for NO _X -storage during the formulation lean engine operation, in the first step NO reacts with oxygen at the active oxidation site on Pt to form NO ₂ . In the second stage the absorption of NO ₂ takes place by the storage material in the form of inorganic nitrates.

When the engine is run intermittently in an enriched state, or when the temperature of the exhaust gas is increased, the nitrate species become thermodynamically unstable and decompose and produce NO or NO ₂ . In concentrated state, such NO _X is reduced to N ₂ by carbon monoxide, hydrogen and hydrocarbons, and this reduction reaction can take place with a reduction catalyst.

Inorganic NO _x -storage components are generally present as oxides, although it is obvious that in the presence of exhaust gases or air comprising CO ₂ and H ₂ O, they may be in the form of carbonates or in the form of hydroxides. In addition, WO 00/21647, filed by the present applicant, the NO _X - it is described that the same may be used to regenerate the trap-specific reaction NO _X.

EP-B-0341832 describes a process for the combustion of particulate matter (PM) in diesel exhaust. The method includes oxidizing NO in exhaust gas to NO ₂ by a catalyst, purifying PM from exhaust gas, and combusting the filtered particulate matter in NO ₂ to 400 ° C. This system is supplied by Johnson Matthey and commercialized as CRT ^® .

EP 0758713A describes an exhaust system for a diesel engine. The system comprises a CRT ^® system described in EP-B-0341832. A heater for intermittently raising the temperature of the exhaust gas to react the carbon collected in the filter with NO ₂ ; And downstream of the CRT ^® filter for removing NO from the exhaust gas NO _X - absorbent or lean NO _X It includes a catalyst. By injecting hydrocarbon fuel into the exhaust gas during the exhaust stroke of one or more engine cylinders, or by injecting the hydrocarbon fuel into the exhaust gas conduit between the engine and the oxidation catalyst, the NO _X -absorber is regenerated, or NO is absorbed on the lean NO _X catalyst. A reducing agent for reducing is supplied.

Injecting a reducing agent into the exhaust gas upstream of the NO _x -absorbent is intended to reduce the oxygen concentration of the exhaust gas, i.e. to concentrate the exhaust gas composition (it does not need to be rich (lambda <1)). to be. However, by injecting the hydrocarbon reducing agent into the exhaust gas of the most upstream of the NO _x -absorbent, the droplets of the liquid hydrocarbon reducing agent evaporate. Also, in a complete gas flow, some amount of reducing agent is needed to remove (through combustion) all of the excess oxygen only before some degree of excess occurs. When the reducing agent is a hydrocarbon fuel such as diesel, this method is expensive in fuel efficiency.

The Applicant has provided a size controlled reducing agent droplet near the upstream surface of the carrier monolith with the NO _x -absorbent, intentionally limiting the vaporization of the injected fluid reducing agent, ie hydrocarbon fuel, thereby reducing the liquid droplet of the reducing agent. It has been found that NO _x -absorbers can be contacted. Thus, the peripheral element is significantly reduced, which can reduce the nitrate stored in the vicinity. Therefore, this configuration NO _X - it is possible to significantly reduce the consumption of reducing agents associated with the absorbent regeneration.

According to a first aspect of the invention, there is provided an article of manufacture comprising: at least one NO _X -absorbent disposed on a single monolithic carrier; Means for injecting liquid reducing agent droplets into the exhaust gas upstream of the at least one carrier; And, in operation, means for controlling the injection of the reducing agent to regenerate the NO _X -absorbent to meet the relevant exhaust standard. The exhaust system have liquid reducing agent solution NO _X - is configured to generate a limited reduction of NO _X in contact with the absorbent.

Techniques for controlling the size of reducing agent droplets in the exhaust system of internal combustion engines are well known to those skilled in the art, and suitable equipment can be selected for this purpose. The parameters considered include the delivery of a reducing agent to the injector head, which can use a common-rail fuel injector in a diesel engine, and the engine speed and / or gas hourly space velocity (GHSV) of the exhaust gases within the system. Accordingly the selection of an appropriate pressure for adjusting the pressure. Injector head designs are well known in the art and can employ electrostatic spray technology or adopt the technical characteristics of fuel burners for domestic boilers. Whatever configuration is chosen, the primary feature of the present invention is that the reducing agent contacts the NO _x -absorbent in the form of liquid reducing agent droplets.

In the first embodiment shown in FIG. 1, the exhaust system comprises a plurality of NO _x − disposed on a single monolithic carrier 42 (each carrier associated with a reducing agent injector 20) arranged in parallel. Absorbents; And in operation, means for continuously contacting at least one of the parallel carriers with the liquid reducing agent droplets while maintaining the plurality of NO _X -absorbers in line with the exhaust stream. The GHSV on each NO _X -trap depends on the relative back pressure of each line, but in general the system will be made such that the configuration is the same in each line, in which case the GHSV will be substantially the same in each line. In which reproduction technology, NO _X in the system - in the NO _X absorbent-absorbent regeneration is performed in series. That is, at any moment at least one line is not sprayed with a reducing agent such that the composition is lean (ie, lambda> 1) when the exhaust gases exiting all NO _x -traps in the system are mixed.

In the second embodiment shown in FIGS. 2A and 2B, the upstream end of the at least one carrier is divided into at least two zones in the fluid flow direction and the exhaust system, in operation, comprises at least one carrier as a whole exhaust gas. And means for continuously contacting at least a portion of the at least two zones with the liquid reducing agent droplets while maintaining in line with the flow. An advantage of this embodiment is that the space for accommodating the system in a vehicle comprising the exhaust system takes less space than in the case of a system comprising a plurality of carrier monoliths each comprising a NO _X -absorbent.

In one configuration of the second embodiment shown in FIG. 3, the means for continuously contacting at least a portion of the two zones with the liquid reducing agent droplets comprises a flap valve disposed at an upstream end of the carrier. A single injector upstream of the flap valve may be used with most of the injected reducing agent directed to a particular zone by the operation of the flap valve. Optionally, each zone divided by the flap valve can be associated with its own injector. The reduced exhaust gas flow in the zone for receiving the reducing agent is to facilitate the playback NO _X absorbent, it can be influenced by the flap valve.

The exhaust system of the first or second embodiment may comprise means for controlling the reducing agent injection by positive feedback to prevent the hydrocarbon reducing agent from unnecessarily exiting into the atmosphere. The control means includes an oxidation catalyst for oxidizing a reducing agent disposed downstream of each NO _x -absorbent carrier; Means for measuring a temperature difference ΔT across the oxidation catalyst; In operation, means for controlling the injection of the liquid reducing agent droplets. The reducing agent droplet injection control means controls the reducing agent injection rate in order to maintain ΔT within a predetermined range, and the exhaust system is configured such that the exhaust gas composition on the oxidation catalyst is lean.

In one embodiment of the exhaust system comprising means for controlling the reducing agent injection, the reducing agent injection rate is reduced when ΔT is too large.

The NO _X -absorbent used in the present invention may include at least one alkali metal, alkaline earth metal or rare earth metal, or a mixture of two or more thereof. Suitable alkali metals can be selected from the group consisting of potassium and cesium. Alkaline earth metals with good effect can be selected from the group consisting of magnesium, calcium, strontium and barium. The rare earth metal can be one or both of lanthanum and yttrium.

In an embodiment, the NO _X -absorber may comprise a catalyst for oxidizing nitrogen monoxide, optionally a platinum group metal such as platinum, and the NO _X -absorber may also be a catalyst for reducing NO _X to N ₂ , such as It may further comprise rhodium.

In a particular embodiment, the sphere at the same time, the control means injects the reducing agent only when the NO _X reduction catalyst is active.

Unless otherwise stated, the catalysts used in the present invention are coated on the high surface area of the carrier monolith made of metal or ceramic or silicon carbide material such as cordierite. A common configuration is a honeycomb monolithic structure through which fluids pass, ranging from 100-600 cpsi in cells per square inch (cpsi), for example 300-400 cpsi (15.5-93.0 cells cm ^-2 , for example 46.5-62.0 cells). cm ^-2 ).

By particle dynamics, liquid reducing agent droplets can pass through a ceramic or metal monolithic carrier through which a conventional fluid can pass, without affecting the NO _X -absorbent held on the wall of the monolithic carrier. In order to increase the likelihood of the reducing agent contacting the NO _X -absorbent, in one embodiment a foam carrier comprising a ceramic or metal foam is used. Alternatively, metallic partial filter substrates comprising internal combustion baffles, such as those disclosed in EP-A-1057519 or WO 03/038248, may be used. According to another embodiment, the NO _X -absorber comprises a conventional ceramic wall flow filter, where the conventionally driven pressure drop must ensure contact with the NO _X stored in the reducing agent droplets. In the latter embodiment, efficient purification of the PM is not important on its own, so that a porous filter can be used, but combined NO _X and as described in JP-B-2722987 (JP-A-06-159037). PM control would be preferred. That is, the filter may be a soot combustion catalyst / NO oxidation catalyst, ie Pt; NO _x -absorbers such as barium oxide; And optionally a NO _X reducing agent catalyst such as rhodium.

In another embodiment, each NO _X -absorbent carrier monolith comprises a particulate filter.

When the oxidation catalyst is coated on a conventional fluid passable monolith and disposed between a reducing agent injector and a NO _X -absorbent carrier, it can serve as an advantage depending on the particle dynamics. Depending on the cell density and the open front region of the monolith, have a reducing agent solution may be passed through the oxidation catalyst without substantial oxidation, NO _X - may be possible to use in order to reduce the NO _X stored in the absorbent. Vaporized hydrocarbon reducing agents, ie gaseous hydrocarbons, will be better oxidized on the oxidation catalyst.

In a separate configuration, the NO _x reduction catalyst and system for delivering the reducing agent described herein are disposed downstream of the configuration described in EP-B-0341832.

According to a second aspect of the invention, there is provided a vehicle comprising an exhaust system according to the invention.

The internal combustion engine may be a lean burn gasoline engine such as a gasoline direct injection engine or a diesel engine. Diesel engines may be light engines or heavy engines classified according to applicable law.

According to a third aspect of the invention, there is provided a method for regenerating a NO _x -absorbent disposed on a single monolithic carrier in an exhaust system of a lean burn internal combustion engine. The method, NO _X - the absorbent liquid brought into contact with the enemy reducing agent solution also includes the step of generating the reduction of NO _X restrictive.

According to one embodiment, the exhaust system comprises a plurality of NO _x -absorbers disposed on a single monolithic carrier 42 arranged in parallel, the method wherein the plurality of NO _x -absorbers are in line with the exhaust stream. Continuously maintaining at least one of the parallel carriers in contact with the liquid reducing agent droplets.

In another embodiment, the method includes the step of continuously contacting a portion of one carrier with a liquid reducing agent droplet while maintaining the carrier in line with the exhaust stream as a whole. Contact of a portion of one carrier with a reducing agent can occur in a reduced exhaust gas stream.

In a separate embodiment, the method comprises oxidizing a reducing agent on an oxidation catalyst disposed downstream of the NO _X -absorbent carrier; Measuring the difference ΔT between the inlet and outlet temperatures of the oxidation catalyst; And adjusting the reducing agent addition rate so that ΔT is maintained within a predetermined range.

Preferably, NO _X-absorbing agent the method comprising: a catalyst for the reduction of NO _X to N ₂ is, only the respective NO _X when the NO _X reduction catalyst active to catalyze the NO _X reduction - Contacting the absorbent carrier with the liquid reducing agent droplets.

1 is a schematic diagram showing an embodiment of an exhaust system according to the present invention.

2A is a schematic diagram showing another embodiment of an exhaust system according to the present invention showing a front view of a NO _X -trap comprising a single carrier monolith indicating the injection zones and injection points of multiple reducing agent injectors at an upstream end of the carrier; .

FIG. 2B is a schematic side view of the single carrier monolith shown in FIG. 2A.

FIG. 3 is a schematic cross-sectional view showing an embodiment of another exhaust system according to the invention, comprising a NO _X -trap with a soot combustion reactor used for the treatment of the exhaust gas of a diesel engine.

4 is a schematic diagram showing an embodiment in which the exhaust system of the present invention operates.

FIG. 5 is a graph showing the upstream air / fuel ratio (AFR) as a function of road speed in the embodiment of FIG. 4.

FIG. 6 is a graph illustrating NO _x measurement in an idle state with respect to the embodiment of FIG. 4. FIG.

FIG. 7 is a graph showing the corresponding system temperature in the idle state for the trace shown in FIG. 6.

FIG. 8 is a graph showing NO _x measurements at 40 mph for the example of FIG. 4. FIG.

FIG. 9 is a graph showing the corresponding temperature measurement at 40 mph for the trace shown in FIG. 8.

FIG. 10 is a graph showing NO _x conversion as a function of road speed for the system of FIG. 4.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings for a deep understanding of the present invention.

An exhaust system 40 according to the invention is shown in FIG. 1. Reference numeral 12 denotes a diesel engine, 14 an exhaust manifold, 16 an exhaust line, and 42 a multiple NO _X -trap catalyst. The multiple NO _x -trap catalysts comprise Pt / Rh and BaO, which are arranged on parallel exhaust lines 44 and are kept on an alumina washcoat on a carrier monolith. Each exhaust line has a respective reducing agent supply means 20 comprising an injector for injecting a large amount of diesel fuel into the exhaust line 16 upstream of the NO _x -trap catalyst 42. The oxidation catalyst 32 (eg, 1 wt% platinum held in the gamma alumina interlayer) is located downstream of the downstream confluence of the exhaust line 44. The thermocouple TC1 detects the temperature of the exhaust gas at the inlet to TC1, and the second thermocouple TC2 is located downstream of the oxidation catalyst 32 to detect the temperature of the exhaust gas at the outlet to TC2. TC1 and TC2 relay the detected temperature to a processor of an engine control unit (ECU, not shown in the figure).

In operation, the system is operated in such a way that the gas is always lean on the oxidation catalyst 32. For example, at any one time at least one line prevents the reducing agent from being injected, so that when the entire NO _x -trap 42 outlet gas stream is mixed, the mixed gas passes over the downstream oxidation catalyst 32. Overall is thin before. NO _X - trap catalyst is at constant below a threshold exhaust gas temperature in the lower light-off temperature (light-off temperature) for generating a catalytic reaction of the NO _X reduction, a reducing agent is not injected. Above this temperature, as the amount of reducing agent increases, the amount of NO _X in the exhaust gas to be reduced increases. Some excess reducing agent slip is oxidized on the oxidation catalyst 32, and the resulting exotherm causes a temperature rise as it passes through the catalyst. This temperature rise is measured as the difference in temperature detected at TC2 and TC1, ie ΔT = TC2-TC1. The control strategy is to adjust the rate of addition of the reducing agent to maintain the measured ΔT at a predetermined value substantially corresponding to the optimum NO _X removal. The flow of reducing agent is increased when ΔT is too small and ΔT is optimally efficient NO _X If it is larger than required for the conversion, it is reduced.

Another embodiment is shown in FIGS. 2A and 2B. The plurality of parallel NO _x -traps 42 in the embodiment of FIG. 1 is replaced with one single NO _x -trap 42A, and three reducing agent feed means 20 are upstream of the NO _x -trap carrier monolith. Equally spaced at the ends is to direct the reducing agent spray onto the juxtaposed zone 45 in front of the carrier monolith. The center of juxtaposed section 45 is defined by injection point 46. This configuration exhibits the same overall effect compared to the case of the embodiment shown in FIG. 1, except that one larger, single NO _x -trap carrier monolith with two or more reducing agent injectors is used. . Since the injector can be operated in a sequential manner, only part of the NO _X -trap is regenerated at any time, and the outlet gas from the part is mixed with the exhaust gas from the other part which is not regenerated, so that the catalyst 32 Provide an overall sparse gas flow for oxidation on

3 shows another embodiment. The exhaust aftertreatment system 80 includes a soot combustion reactor 120. The inlet of the soot combustion reactor 120 is connected to an exhaust manifold of a diesel engine (not shown in the figure). The reactor 120 includes an oxidation catalyst 122 composed of an alumina based intermediate layer and a ceramic honeycomb having Pt in an upstream portion thereof. A downstream portion of reactor 120 includes a wall flow filter 124, which is comprised of a filter grade ceramic honeycomb. The passage can be selectively plugged or unplugged at the inlet end and can be selectively plugged at the outlet end. The passage plugged at the inlet end is unplugged at the outlet end and vice versa. The configuration of the oxidation catalyst for oxidizing NO to NO ₂ for combustion of PM on the downstream filter is described in EP-B-0341832, which is known as CRT ^® . From the outlet end of the reactor 120, the plenum 126 continues as an operating chamber of the flap valves 128X, Y, Z at the inlet of the NO _X -trap vessel 130. The vessel 130 includes NO _X -traps 131X, Y composed of a fluid-through ceramic honeycomb monolith carrier having barium oxide and an alumina interlayer comprising metals Pt and Rh. The flap valves 128X, Y, Z extend radially across the surface of the reactor 130 and are sealed to prevent the gas from passing through the surface of the NO _X -trap 131. Each zone (X, Y) on each side of the reactor 130 of the valve 128 is equipped with reducing agent injectors 132X, Y. In the completed reactor 130 as shown, the valve 128 is in the center position Z. Valve positions (X, Y) are shown as inserts. The reactor 130 is provided with an outlet 134 for guiding the atmosphere or subsequent processing. Preferably, in the configuration shown in FIG. 1, the flow rate in two separate halves of the reactor 130 is controlled to provide a pure lean composition, and the mixture is passed through an oxidation catalyst.

During normal operation of the system, steam (H ₂ O (g)), nitrogen molecules (N ₂ ), oxygen (O ₂ ), carbon dioxide (CO ₂ ), unburned hydrocarbons (HC), carbon monoxide (CO), Exhaust gases comprising nitrogen oxides (NO _X ) and incident phase materials (PM) are in contact with catalyst 122 at 300 ° C., for example, NO is oxidized to NO ₂ , and some of HC and CO to steam and CO ₂ . Is oxidized. Thereafter, entering the filter 124, most of the PM is collected and combusted by reacting with NO ₂ formed in the catalyst 122, and may be reacted and combusted with O ₂ . The PM-free gas is treated in one of three modes:

128Z: Both the NO _X -trap zones 130X, 130Y absorb NO _X.

128X: Zone 131X receives a fraction of the gas leaving plenum 126 and diesel fuel injection at 132X. It goes through regeneration and the effluent recombines with that of zone 130Y. Zone 131Y receives most of the gas, absorbs NO _x , and passes the effluent to the atmosphere at 134.

128Y: Zone 131Y performs the process described at 128X.

Engine treatment system (not shown in the figure) is NO _X - is changed to a trap (131Y) empty case have a volume in the zone (X) zone (Y) in that is absorbs NO _X, also holds for the station do.

(Example)

The following examples are merely illustrative.

A single-floor bus exhaust system 50 (see FIG. 4) comprising an engine turbo 52 of the type suitable for a 6 liter turbocharged engine and meeting the European Stage 1 emission limits, allows the exhaust gas to pass through three parallel legs 56. And a three-way splitter 54 for switching to one of In each leg the exhaust flows at the same speed. Each leg 56 includes a chamber 58 that includes an oxidation catalyst 60 followed by a NO _X -trap 62. Gas flow is NO _X - are combined downstream of the trap, before the exhaust gas is directly passed into the atmosphere NO _X - the total exhaust gas flow of "clean-up (clean up) in order to remove unburned hydrocarbons (HC) coming out of the trap Pass through oxidation catalyst 64. A fuel injector 66 comprising a fuel solenoid 68 is disposed in front of each oxidation catalyst 60 and a NO _X sensor 69 is disposed in front of the exhaust splitter 54 and combined NO _x / air. The fuel ratio sensor 70 is disposed behind the NO _x -trap, and thermocouples T1, T2, T3, and T4 for measuring temperature are disposed before and after the oxidation catalyst 60 and at the reactor outlet. The oxidation catalyst 60 and the NO _x -trap 62 are coated on a ceramic fluid passage and monolith with a 400 cell in ^-2 (62 cell cm ^-2 ) and 0.06 in (0.15 mm) wall thickness, respectively. The oxidation catalyst 60 is 5.66 inches (144 mm) in diameter, 3 inches (76 mm) in length, and has a volume of 75.5 inches ³ (1.24 liters). The diameter of the NO _x -trap 62 is the same as the diameter of the oxidation catalyst 60 but is 6 inches (152 mm) in length. The "clean up" catalyst 64 has a diameter of 10.5 inches (267 mm), a length of 3 inches (76 mm), and a volume of 260 inches ³ (4.26 liters).

The experiment described in this embodiment is performed using only one leg of the exhaust split. The vehicle is driven using diesel fuel containing 50 ppm of sulfur and is driven at constant idling speeds of 10, 20, 30 and 40 mph during operation. Fuel is injected at each point with the air-fuel ratio during the injection determined as shown in FIG. The combination of time and duration (two-second injection, once per minute per leg) determines the exhaust gas temperature (temperature to maintain the NO _X -trap within the dynamic temperature window) and NO _X. Empirically selected to achieve the best combination between transforms. At the same time, the NO _x -emissions before and after the system together with the temperature profile are measured.

In FIG. 5, the square wave represents the ideal air / fuel ratio behind the injector and in front of the catalyst. The exhaust gas is normally lean, but momentarily becomes rich during injection. The 'excess' air / fuel ratio (based on injected fuel volume, exhaust stoichiometry and exhaust flow rate), calculated as a function of road speed, is represented by the curve. If the stoichiometric air / fuel ratio is 14.7: 1, it has been found that the injector cannot produce complete excess mixing at speeds faster than about 6 mph. The measured air / fuel ratio used in the figures below is taken from the post-NO _X -trap sensor. Because of the chemical motion and absorption inside the catalyst system, no contoured square wave is achieved.

6 is NO _x from the engine. Exhaust (ppm), and NO _X - During the idle state with the air-fuel ratio measured after the NO _X trap - trap after the NO _X Exhaust (ppm). 7 shows temperature traces during the same time. In FIG. 6, once fuel is injected at the start of idle time, the air-fuel ratio can be observed to fall from lean to excess as predicted from the expectation in FIG. 5, with good NO _X conversion after initial NO _X breakthrough. This is observed. Over time, the air-fuel ratio remains sparse throughout the injection operation, but good NO _X conversion remains. The exotherm T2 generated over the oxidation catalyst allows the temperature of the NO _X -trap to be maintained between operating windows of 220-550 ° C. In addition, the exotherm T3 is recorded across the NO _x -trap, some of which are generated as the unreacted gas reducing agent is burned from the oxidation catalyst. These results are interpreted to mean that some of the exotherm arises from the combustion of unburned fuel droplets that react on the surface of the NO _X -trap over time in the engine idle state. This is because the system inlet temperature is insufficient to vaporize the incoming fuel, and also the air / fuel ratio peak measured by the rear sensor, indicating a series of settling, vaporization and thus sequential oxidation of the fuel droplets. ) Is not displayed correctly and becomes rounder. Some of the excess thus caused also serves to maintain the observed NO _x -trap operating efficiency.

Results of testing with a bus maintained at a constant speed of 40 mph are shown in FIGS. 8 and 9. The exhaust flow rate is higher here, but the same injection flow rate is used in the idle state, and the exhaust is required to be in a lean state throughout the injection time (see FIG. 5). However, apart from the breakthrough spike when the fuel is first injected, NO _X is reduced over the remaining operating time, although not as efficient as in the idle state. The exotherm T3 above the exothermic T2 is sometimes lower than in the idle state, but because of the increased heat capacity of the exhaust gas, this is very important. Therefore, an exothermic reaction is still taking place, and it is also believed that this phenomenon is because some of the unburned fuel droplets are carried through the oxidation catalyst and burned on the NO _X -trap. It is expected that fuel droplets will persist despite the high inlet temperature of the oxidation catalyst, as more exhaust flow rates are oxidized, as indicated by the marked exotherm measured through the NO _X -trap and the trap regeneration observed in the apparently sparse state. This is because the droplets will be transported through the catalyst.

FIG. 10 shows the trend in average NO _x conversion efficiency calculated for the system as a function of speed. FIG. 5 shows that excess exhaust gas does not occur above about 6 mph, and a good NO _X conversion can be obtained in lean state over a wide speed range. This is particularly relevant in the range of 30 mph from idle (which is the most common operating range for urban buses).

Claims (26)

In the exhaust system (40; 50; 80) for the lean burn internal combustion engine (12), At least one NO _X -absorbent disposed on a single monolithic carrier 42; 42A; 60; 130; Means (20; 66; 68; 132) comprising an injector for injecting liquid reducing agent droplets into the exhaust gas upstream of the at least one carrier; And In operation, means for controlling the injection of the reducing agent to regenerate the NO _X -absorbent so as to meet the relevant exhaust standard; The exhaust system, characterized in that in contact with the sorbent that causes the reduction of the localized NO _X - have the liquid reducing agent solution NO _X. The method of claim 1, A plurality of NO _x -absorbers disposed on a single monolithic carrier 42 each associated with and parallel to the reducing agent injector 20; And In operation, means for continuously contacting at least one of the parallel carriers with the liquid reducing agent droplets while maintaining the plurality of NO _x -absorbers in line with the exhaust gas stream. The method of claim 1, An upstream end of at least one carrier 42A; 130 is divided into at least two zones 45 (131X, 131Y) in the direction of fluid flow, Exhaust system, In operation, means for continuously contacting at least a portion of the at least two zones with the liquid reducing agent droplets while maintaining at least one carrier in line with the exhaust gas stream as a whole. The method of claim 3, wherein Said means for continuously contacting at least a portion of at least two zones with liquid reducing agent droplets comprises a flap valve (128) disposed at an upstream end of the carrier. The method according to claim 3 or 4, An exhaust system (20; 132X, 132Y) associated with each said zone. The method according to any one of claims 2 to 5, An oxidation catalyst 32 for oxidizing a reducing agent disposed downstream of each NO _x -absorbent carrier; Means (TC1, TC2) for measuring a temperature difference ΔT across the oxidation catalyst; In operation, means for controlling the injection of the liquid reducing agent droplets; The reducing agent droplet control means for controlling the reducing agent injection rate in order to maintain ΔT within a predetermined range, And an exhaust gas composition on the oxidation catalyst is lean. The method of claim 6, The reducing agent injection rate is reduced when ΔT is too large. The method according to any one of claims 1 to 7, An NO _x -absorbent comprises at least one compound of an alkali metal, an alkaline earth metal or a rare earth metal, or a mixture of two or more thereof. The method of claim 8, And the alkali metal is selected from the group consisting of potassium and cesium. The method of claim 8, And an alkaline earth metal is selected from the group consisting of magnesium, calcium, strontium and barium. The method of claim 8, And the rare earth metal is selected from the group consisting of lanthanum and yttrium. The method according to any one of claims 1 to 11, NO _x -absorbent comprises a catalyst for oxidizing nitrogen monoxide, optionally a platinum group metal such as platinum. The method of claim 12, NO _x -absorbent comprises a catalyst for reducing NO _x to N ₂ , such as rhodium. The method according to claim 12 or 13, Obtain at the same time, the exhaust system comprises a control means for injecting a reducing agent only when the NO _X reduction catalyst is active. The method according to any one of claims 1 to 14, NO _x -absorbent carrier monolith comprises an ceramic or metal foam. The method according to any one of claims 1 to 14, NO _x -absorbent carrier monolith comprises an incidence filter. The method according to any one of claims 1 to 16, And an oxidation catalyst disposed between the injector and the NO _x -absorbent carrier monolith. 18. A vehicle comprising an exhaust system according to any of the preceding claims. The method of claim 18, A vehicle comprising a diesel engine. A method of regenerating a NO _X -absorbent disposed on a single monolithic carrier (42; 42A; 60; 130) in an exhaust system (40; 50; 80) of a lean burn internal combustion engine, NO _X - A method of reproducing a sorbent-absorbing agent to the liquid reducing agent liquid contact enemy NO _X, characterized in that it comprises the step of generating a reduction of the localized NO _X. The method of claim 20, The exhaust system comprises a plurality of NO _x -absorbers disposed on a single monolithic carrier 42 arranged in parallel, A method of reproducing a sorbent-NO _X that the while the sorbent maintains the line status to the exhaust stream, at least one parallel support characterized in that it comprises the step of contacting is continuously enemy liquid reducing agent solution - a plurality of NO _X. The method of claim 20, Method for reproducing the absorbent - while the carrier is held to the line state to the exhaust gas flow as a whole, a part of NO _X to the carrier (42A) characterized in that it comprises the step of contacting is continuously enemy liquid reducing agent solution. The method of claim 22, Contacting a portion of the carrier to the enemy liquid reducing agent solution are continuously NO _X, it characterized in that the reduction takes place in the exhaust gas flow-method for reproducing absorbent. The method according to any one of claims 21 to 23, Oxidizing a reducing agent on an oxidation catalyst 32 disposed downstream of the NO _X -absorbent carrier; Measuring the difference ΔT between the inlet and outlet temperatures of the oxidation catalyst; And A method of reproducing a sorbent - NO _X, characterized in that it comprises a; ΔT the step of controlling the reducing agent addition rate is kept within a predetermined range. The method according to any one of claims 20 to 23, The NO _X -absorbent comprises a catalyst for reducing NO _X to N ₂ , A method of reproducing a sorbent-NO _X absorbent to a carrier, characterized in that it comprises the step of contacting the enemy liquid reducing agent solution, each of the NO _X only when the NO _X reduction catalyst activated in order to catalyze the reduction of NO _X. The method according to any one of claims 20 to 25, The reducing agent, NO _X, characterized in that it comprises a hydrocarbon fuel, such as for supplying power to the engine - a way to play the absorbent.

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