Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/02—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust
  • F01N3/021—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters
  • F01N3/033—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters in combination with other devices
  • F01N3/035—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters in combination with other devices with catalytic reactors, e.g. catalysed diesel particulate filters
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
  • F01N13/009—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more separate purifying devices arranged in series
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
  • F01N13/011—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more purifying devices arranged in parallel
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/02—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust
  • F01N3/021—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters
  • F01N3/023—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters using means for regenerating the filters, e.g. by burning trapped particles
  • F01N3/0231—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters using means for regenerating the filters, e.g. by burning trapped particles using special exhaust apparatus upstream of the filter for producing nitrogen dioxide, e.g. for continuous filter regeneration systems [CRT]
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
  • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
  • F01N3/0814—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents combined with catalytic converters, e.g. NOx absorption/storage reduction catalysts
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
  • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
  • F01N3/0821—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents combined with particulate filters
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
  • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
  • F01N3/0828—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents characterised by the absorbed or adsorbed substances
  • F01N3/0842—Nitrogen oxides
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
  • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
  • F01N3/0871—Regulation of absorbents or adsorbents, e.g. purging
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
  • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
  • F01N3/0871—Regulation of absorbents or adsorbents, e.g. purging
  • F01N3/0878—Bypassing absorbents or adsorbents
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N2610/00—Adding substances to exhaust gases
  • F01N2610/03—Adding substances to exhaust gases the substance being hydrocarbons, e.g. engine fuel

Definitions

• the present invention relates to an exhaust system for a lean-burn internal combustion engine such as a diesel engine comprising an absorbent for nitrogen oxides (NO x ).
• a lean-burn internal combustion engine such as a diesel engine comprising an absorbent for nitrogen oxides (NO x ).
• the invention relates to a method of regenerating such a NO x - absorbent.
• Exhaust systems for vehicular lean-burn internal combustion engines comprising a device for absorbing nitrogen oxides (NO x ) from lean exhaust gas and releasing the stored NO x in an atmosphere containing less oxygen for reduction to dinitrogen (N 2 ) are known from, for example, EP 0560991 (incorporated herein by reference).
• NO ⁇ -absorbents are typically associated with a catalyst for oxidising nitrogen monoxide (NO) to nitrogen dioxide (NO 2 ), e.g. platinum (Pt), and, optionally, also a catalyst such as rhodium, for reducing NO x to N 2 with a suitable reductant, e.g. a hydrocarbon.
• a catalyst comprising the NO x -absorbent and a NO oxidation catalyst and optional NO x reduction catalyst is often called a lean NO x -trap or simply a NO x -trap.
• NO x -absorbents in a typical NO x -trap formulation can include compounds of alkali metals, e.g. potassium and/or caesium; compounds of alkaline earth metals, such as barium or strontium; and/or compounds of rare-earth metals, typically lanthanum and/or yttrium.
• alkali metals e.g. potassium and/or caesium
• alkaline earth metals such as barium or strontium
• rare-earth metals typically lanthanum and/or yttrium.
• One mechanism commonly given for NO x -storage during lean engine operation for this formulation is that, in a first step, the NO reacts with oxygen on active oxidation sites on the Pt to form NO 2 .
• the second step involves adsorption of the NO 2 by the storage material in the form of an inorganic nitrate.
• the nitrate species become thermodynamically unstable and decompose, producing NO or NO 2 .
• these NO x are reduced by carbon monoxide, hydrogen and hydrocarbons to N 2 , which can take place over the reduction catalyst.
• the inorganic NO x -storage component is typically present as an oxide, it is understood that in the presence of air or exhaust gas containing CO 2 and H 2 O it may also be in the form of the carbonate or possibly the hydroxide.
• NO x -specific reactants can be used to regenerate a NO x -trap.
• EP-B-0341832 (incorporated herein by reference) describes a process for combusting particulate matter (PM) in diesel exhaust gas, which method comprising oxidising NO in the exhaust gas to NO 2 on a catalyst, filtering the PM from the exhaust gas and combusting the filtered PM in the NO 2 at up to 400°C.
• PM particulate matter
• EP 0758713 A discloses an exhaust system for a diesel engine, which system comprising a CRT ® system as disclosed in EP-B-0341832, a heater for intermittently raising the exhaust gas temperature to react NO 2 with carbon collected on the filter and a NO x -absorbent or a lean NO x catalyst downstream of the CRT ® filter for removing NO in the exhaust gas.
• the NO x -absorbent is regenerated, or reductant for reducing NO on the lean NO x catalyst is supplied, by introducing hydrocarbon fuel into the exhaust gas either 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 intention of injecting reductant into the exhaust gas upstream of a NO x -absorbent is to reduce the oxygen concentration of the exhaust gas, i.e. to enrich, but not necessarily to make rich (lambda ⁇ 1), the exhaust gas composition.
• hydrocarbon reductant is injected into the exhaust gas far upstream of the NO x -absorbent, droplets of the liquid hydrocarbon reductant evaporate.
• a significant amount of reductant is required merely to remove all the excess oxygen (through combustion) before any degree of richness is obtained.
• the reductant is a hydrocarbon fuel such as Diesel, this approach is costly on fuel economy.
• the invention provides an exhaust system for a lean- burn internal combustion engine comprising at least one NO x -absorbent disposed on a unitary monolith substrate, means comprising an injector for injecting droplets of a liquid reductant into exhaust gas upstream of the at least one substrate and means, when in use, for controlling the injection of reductant in order to regenerate the NO x -absorbent thereby to meet a relevant emission standard, the arrangement being such that droplets of the liquid reductant contact the NO x -absorbent thereby causing localised reduction of NO x .
• the skilled person is well aware of techniques for controlling the droplet size of reductants in exhaust systems of internal combustion engines and the appropriate equipment can be selected for the desired purpose.
• Parameters for consideration include selection of appropriate pressure for delivering the reductant to the injector head, which can use common-rail fuel injectors in diesel engines, and modulating the pressure depending on engine speed and/or gas hourly space velocity of exhaust gas in the system.
• Design of injector heads is well known from parallel arts and can adopt use of electrostatic spray techniques, or aspects of technology from fuel burners for household boilers etc. Whatever arrangement is selected, the overriding feature of the invention is that the reductant contacts the NO x -absorbent in the form of droplets of liquid reductant.
• the exhaust system comprises a plurality of NO x -absorbents disposed on unitary monolith substrates arranged in parallel, each substrate associated with a reductant injector and means, when in use, for successively contacting at least one of the parallel substrates with droplets of liquid reductant whilst the plurality of NO x -absorbents remain in-line to exhaust gas flow.
• the gas hourly space velocity (GHSV) over each NO x -trap is dependent on the relative backpressure in each line, but normally the system will be set up so that the arrangement is the same in each, in which case the GHSV will be substantially the same in each line.
• NO x -absorbent regeneration is conducted in series in the NO x -absorbents in the system, i.e. at any instant, at least one line is not having reductant injected, so that when the exhaust gas exiting all NO x -traps in the system is mixed, its composition is lean, i.e. lambda >1.
• an upstream end of the at least one substrate is subdivided in the direction of fluid flow into at least two zones and the system comprises means, when in use, for contacting successively a fraction of the at least two zones with droplets of liquid reductant whilst the at least one substrate as a whole remains in-line to exhaust gas flow.
• the means for contacting the fraction of the at least two zones with droplets of liquid reductant comprises a flap valve disposed at the upstream end of the substrate.
• a single injector upstream of the flap valve can be used with a majority of the injected reductant being directed to a particular zone by actuation of the flap valve.
• each zone divided by the flap valve can be associated with its own injector.
• Reduced exhaust gas flow in the fraction receiving reductant can promote NO x absorbent regeneration and can be effected by flap valve actuation.
• the exhaust system of the first or second embodiment can include means for controlling, by positive feed-back, reductant injection in order to prevent unnecessary slip of hydrocarbon reductant to atmosphere.
• the control means comprises an oxidation catalyst for oxidising the reductant disposed downstream of the or each NO x -absorbent substrate, means for determining a temperature difference ( ⁇ T) across the oxidation catalyst and means, when in use, for controlling injection of droplets of liquid reductant, wherein the reductant droplet injection control means controls the rate of reductant injection to maintain ⁇ T within a pre-determined range, wherein the system is configured so that the exhaust gas composition over the oxidation catalyst is lean.
• an exhaust system comprising the means for controlling reductant injection, wherein the rate of reductant injection is decreased if ⁇ T is too large.
• the NO x -absorbent for use in the present invention can comprise at least one alkali metal, alkaline earth metal or rare-earth metal or a mixture of any two or more thereof.
• Suitable alkali metals can be selected from the group consisting of potassium and caesium; efficacious alkaline-earth metals can be selected from the group consisting of magnesium, calcium, strontium and barium; and the rare-earth metal can be one or both of lanthanum and yttrium.
• the NO x -absorbent can comprise a catalyst for oxidising nitrogen monoxide, optionally a platinum group metal such as platinum and can further comprise a catalyst for reducing NO x to N 2 , such as rhodium.
• the control means when in use, injects reductant only when the NO x reduction catalyst is active.
• the catalysts for use in the present invention are coated on high surface area substrate monoliths made from metal or ceramic or silicon carbide, e.g. cordierite, materials.
• a common arrangement is a honeycomb, flowthrough monolith structure of from 100-600 cells per square inch (cpsi) such as 300-400 cpsi
• Particle dynamics can cause the droplets of liquid reductant to pass through a conventional flow-through ceramic or metal monolith substrate without impinging on the NO x -absorbent carried on the walls thereof.
• a foam substrate comprising a ceramic or metal foam is used.
• An alternative embodiment utilises metallic partial filter substrates including internal baffles, such as disclosed in EP-A- 1057519 or WO 03/038248 (both incorporated herein by reference).
• the NO x -absorbent comprises a conventional ceramic wall-flow filter; here pressure-drop driven convention should ensure that reductant droplets contact stored NO x .
• the filter includes a soot combustion catalyst/NO oxidation catalyst e.g. Pt, a NO x -absorbent such as barium oxide and, optionally, a NO x reduction catalyst e.g. rhodium.
• a soot combustion catalyst/NO oxidation catalyst e.g. Pt
• a NO x -absorbent such as barium oxide
• a NO x reduction catalyst e.g. rhodium
• the or each NO x -absorbent substrate monolith comprises a particulate filter.
• Advantage can also be made of particle dynamics when an oxidation catalyst is coated on a conventional flow-through monolith and is disposed between the reductant injector and the NO x -absorbent substrate.
• reductant droplets can pass through the oxidation catalyst substantially without oxidation and be available for reducing stored NO x in the NO x -absorbent.
• Evaporated hydrocarbon reductant i.e. gaseous hydrocarbon, is more likely to be oxidised on an oxidation catalyst.
• the NO x reduction catalysts and systems for delivering reductant described herein are disposed downstream of the arrangement described in EP-B-0341832, mentioned hereinabove.
• the invention provides a vehicle comprising an exhaust system according to the invention.
• the internal combustion engine can be a diesel or lean-burn gasoline engine, such as a gasoline direct injection engine.
• the diesel engine can be a light-duty engine or a heavy-duty engine, as defined by the relevant legislation.
• the invention provides a method of regenerating a NO x -absorbent disposed on a unitary monolith substrate in the exhaust system of a lean- bum internal combustion engine, which method comprising contacting the NO x - absorbent with droplets of a liquid reductant thereby causing localised reduction of NO x .
• the exhaust system comprises a plurality of NO x -absorbents disposed on unitary monolith substrates arranged in parallel
• the method comprises contacting successively at least one of the parallel substrates with droplets of liquid reductant whilst the plurality of NO x absorbents remain in-line to exhaust gas flow.
• the method comprises contacting successively a fraction of a single substrate with the liquid reductant droplets while the substrate as a whole remains in-line to exhaust gas flow. Where only a fraction of a single substrate is contacted with the reductant, this can be done at reduced exhaust gas flow.
• the method provides the step of oxidising the reductant over an oxidation catalyst located downstream of the NO x -absorbent substrate, determining the difference between the inlet and the outlet temperatures ( ⁇ T) of the oxidation catalyst and adjusting the rate of reductant injection so that ⁇ T is within a pre-determined range.
• the method comprises contacting the or each substrate with liquid reductant droplets only when the NO x reduction catalyst is active for catalysing NO x reduction.
• Figure 1 is a schematic of one embodiment of an exhaust system according to the invention
• Figure 2A is a schematic of another embodiment of an exhaust system according to the invention showing an end-on view of a NO x -trap comprising a unitary substrate monolith showing the injection points and spray zones of multiple reductant injectors at the upstream end of the substrate;
• Figure 2B is a schematic side view of the unitary substrate monolith shown in Figure 2A;
• Figure 3 is a schematic sectional view of an embodiment of another exhaust system embodiment of the invention including a NO x -trap in combination with a soot combustion reactor for use in treating the exhaust gas of a diesel engine;
• Figure 4 is a schematic of a working exhaust system embodiment of the invention.
• Figure 5 is a graph showing the upstream Air/Fuel Ratio (AFR) as a function of road speed in the embodiment of Figure 4;
• Figure 6 is a graph showing NO x measurements at the idle condition for the embodiment of Figure 4.
• Figure 7 is a graph showing the corresponding system temperatures at the idle condition for the trace shown in Figure 6;
• Figure 8 is a graph showing NO x measurements at 40mph for the embodiment of
• Figure 9 is a graph showing the corresponding temperature measurements at 40mph for the trace shown in Figure 8.
• Figure 10 is a graph showing the NO x conversion as a function of road speed for the system of Figure 4.
• FIG. 1 An exhaust system, generally referenced as 40, according to an embodiment of the invention is shown in Figure 1, wherein 12 represents a diesel engine, 14 the exhaust manifold, 16 the exhaust line and 42 multiple NO x -trap catalysts comprising Pt/Rh and
• each line having its own reductant supply means 20 including an injector for injecting a quantity of diesel fuel into the exhaust line 16 upstream of NO x -trap catalysts 42.
• Oxidation catalyst 32 e.g. lwt% platinum supported on a gamma-alumina washcoat, is located downstream of the downstream join of exhaust lines 44.
• Thermocouple TCI detects the temperature of the exhaust gas at the inlet to TCI and a second thermocouple TC2 is located downstream of oxidation catalyst 32 to detect the temperature of exhaust gas at the outlet thereto. TCI and TC2 relay the detected temperatures to a processor in the engine control unit (ECU (not shown)).
• the system is operated in such a way as to ensure the gas is always lean over the oxidation catalyst 32.
• at least one line is not having reductant injected, so when the total NO x -trap 42 exit gas streams are mixed, the resulting gas is overall lean before passing over the downstream oxidation catalyst 32.
• the control strategy is to adjust the rate of reductant addition to keep the measured ⁇ T at substantially a pre-determined value corresponding to optimum
• the reductant flow is increased if ⁇ T is too small, or decreased if ⁇ T is larger than desired for optimum efficient NO x conversion.
• FIG. 2 A and 2B Another embodiment is shown in Figures 2 A and 2B, wherein the plurality of parallel NO x -traps 42 of the Figure 1 embodiment are replaced by a single unitary NO x -trap 42A and three reductant supply means 20 are disposed equidistantly at the upstream end of the NO x -trap substrate monolith and directing a reductant spray onto juxtaposed zones 45 on the front face of the substrate monolith whose centres are defined by injection points 46.
• This arrangement provides the same overall effect as the embodiment illustrated in Figure 1 but using a larger single, i.e. unitary NO x -trap substrate equipped with two or more reductant injectors.
• the injectors can be operated in a sequential manner so at any one time only part of the NO x -trap is undergoing regeneration, and exit gas from this part is mixed with exhaust gas from parts not being regenerated to provide an overall lean gas stream for oxidation on catalyst 32.
• an exhaust gas aftertreatment system 80 comprises a soot combustion reactor 120 the inlet of which is connected to the exhaust manifold of a diesel engine (not shown).
• Reactor 120 at its upstream portion contains oxidation catalyst 122 consisting of a ceramic honeycomb carrying an alumina-based washcoat and Pt.
• oxidation catalyst 122 consisting of a ceramic honeycomb carrying an alumina-based washcoat and Pt.
• wall-flow filter 124 consisting of filter-grade ceramic honeycomb, the passages of which are alternately plugged and unplugged at the inlet end and alternately plugged at the outlet end, wherein passages plugged at the inlet end are unplugged at the outlet end, and vice versa.
• oxidation catalyst for oxidising NO to NO 2 for combustion of PM on a downstream filter
• CRT ® Such an arrangement of oxidation catalyst for oxidising NO to NO 2 for combustion of PM on a downstream filter is described in EP-B-0341832 and the arrangement is known as the CRT ® .
• From the outlet end of reactor 120 plenum 126 continues as the operating chamber of flap valve 128X,Y, Z at the inlet of NO x -trap vessel 130.
• Vessel 130 contains NO x -trap 131X,Y consisting of a flowthrough ceramic honeycomb monolith substrate carrying an alumina washcoat containing barium oxide and metallic Pt and Rh.
• the fulcrum of flap valve 128X,Y,Z is mounted on partition 129 which extends diametrically across the face of reactor 130 and is gas-tightly sealed to the face of NO x -trap 131.
• valve 128 Each region X,Y of reactor 130 either side of valve 128 is provided with reactant injector 132X,Y.
• valve 128 In the complete reactor 130 as shown, valve 128 is in the central position Z. Valve positions X and Y are shown as insets.
• Reactor 130 is formed with outlet 134, leading to atmosphere or to further treatment. Preferably, rates of flow in the two halves of reactor 130 are controlled to give a net lean composition and the mixture is passed over an oxicat, in the arrangement shown in Figure 1.
• the exhaust gas comprising steam (H 2 O (g)), dinitrogen (N 2 ), oxygen (O 2 ), carbon dioxide (CO 2 ), unbumed hydrocarbon fuel (HC), carbon monoxide (CO), nitrogen oxides (NO x ) and particulate matter (PM), at e.g. 300°C contacts catalyst 122 over which NO is oxidised to NO 2 and some of the HC and CO are oxidised to steam and CO 2 . It then enters filter 124 on which most of the PM is collected and combusted by reaction with the NO 2 formed in catalyst 122 and possibly with O 2 .
• steam H 2 O (g)
• dinitrogen N 2
• oxygen oxygen
• CO 2 unbumed hydrocarbon fuel
• CO carbon monoxide
• NO x nitrogen oxides
• PM particulate matter
• the PM-freed gas then undergoes treatment in one of the 3 modes: 128Z: NO x -tra ⁇ regions 130X and 130Y both absorb (or adsorb) NO x ; 128X: region 13 IX receives a small fraction of the gas leaving plenum 126 and injection of diesel fuel at 132X. It undergoes regeneration, and its effluent is reunited with that of region 130Y; region 131Y receives the major portion of the gas, absorbs NO x and passes its effluent to atmosphere at 134; 128Y: region 131Y performs the duty described at 128X.
• the engine management system (not shown) changes from region X to region Y when NO x -trap 131 Y has free capacity to absorb NO x ; and vice versa.
• the exhaust system (50) (shown in Figure 4) of a single deck bus fitted with a 6 litre turbocharged engine and comprising engine turbo (52), type approved to European Stage 1 emission limits, was modified to incorporate a three-way splitter (54) for diverting the exhaust gas into one of three parallel legs (56), the exhaust gas flow in each leg being of equal velocity flow.
• Each leg (56) comprised a chamber (58) containing an oxidation catalyst (60) followed by a NO x -trap (62).
• a fuel injector (66) comprising a fuel solenoid (68) was sited in front of each oxidation catalyst (60), a NO x sensor (69) in front of the exhaust splitter (54), combined NO x /air fuel ratio sensors (70) behind the NO x -traps and thermocouples (Tl, T2, T3, T4) measuring temperatures in front of and behind the oxidation catalysts (60) and at the exit of the reactors.
• the oxidation catalysts (60) and the NO x traps (62) were each coated on ceramic flow- through monoliths at 400 cells in '2 (62 cells cm “2 ) and 0.06 in (0.15mm) wall thickness.
• the oxidation catalysts (60) were 5.66 in (144mm) diameter x 3 in (76mm) and volume 75.5 in 3 (1.24 litre), the NO x -traps (62) were the same diameter but 6 in (152mm) long and the "clean up" catalyst (64) 10.5 in (267mm) diameter x 3 in (76mm) long and volume 260 in 3 (4.26 litres).
• the experiments described here were conducted using one leg of the split exhaust only.
• the vehicle was operated using diesel fuel containing 50ppm sulphur and run at steady speeds of idle, 10, 20, 30 and 40 mph for periods of time; fuel was injected at each of these points and the air fuel ratio during injection determined as shown in Figure 5.
• the combination of time and duration (2 seconds injection, one per minute per leg) was selected empirically as it gave the best combination of exhaust gas temperatures (to maintain the NO x -trap within an active temperature window) and NO x conversion. Simultaneously the NO x emissions pre- and post- the system together with the temperature profiles were measured.
• the square wave represents the idealised air/fuel ratio after the injector and before the front face of the catalyst.
• the exhaust gas mixture is normally lean, but becomes instantaneously richer during injection.
• the calculated 'rich' air/fuel ratio (on the basis of the fuel volume injected, exhaust stoichiometry and exhaust flow rate) as a function of road speed is represented by the curve. It was found that, if the stoichiometric air/fuel ratio is 14.7:1, then the injector was unable to create a truly rich mixture at speeds greater than about 6mph.
• the measured air/fuel ratio used in later Figures is taken from a post-NO x -trap sensor. Because of the absorption and chemical activity within the catalyst system, the well-defined shape of the square wave is lost.
• Figure 6 shows the NO x emissions (ppm) from the engine and after the NO x -trap for the idle condition together with the air fuel ratio measured after the NO x - trap.
• Figure 7 shows the temperature traces for the same period. From Figure 6 it is seen that when fuel is injected at the start of the idle period, the air fuel ratio drops from lean to rich as expected from the predictions in Figure 5 and, after the initial NO x breakthrough, good NO x conversion is seen. With time, the air fuel ratio remains lean throughout the injection event but good NO x conversion is still maintained.
• the exotherm (T2) generated over the oxidation catalyst helps maintain the temperature of the NO x -trap within its operating window of 220-550°C.

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