Abstract:
A lean NOx trap operating in a lean NOx exhaust stream is progressively regenerated, sector to sector, for efficient operation of the trap and conservation of reducing gas used in the regeneration. A reductant gas distributor is inserted in the exhaust upstream of the trap. Reductant gas is carried into the exhaust duct through a conduit portion and the reductant gas stream diverged and directed through a distributor into selected cells of the trap. The NOx in the selected cells is reduced to N 2  while lean NOx exhaust continues to flow through the remainder of the trap. By relative movement of the reductant gas distributor and the trap, the entire trap is progressively and continually regenerated while still preforming as a NOx trap.

Description:
TECHNICAL FIELD 
     This invention pertains to the temporary trapping and subsequent reduction of nitrogen oxides (NOx) in the exhaust gas from a diesel engine or other lean-burn internal combustion engine. More specifically, this invention pertains to the progressive and continuous introduction of a reductant gas into the inlet of exhaust flow-through lean NOx traps (LNT) for catalytic reduction of the NOx to nitrogen. 
     BACKGROUND OF THE INVENTION 
     The mass ratio of air-to fuel (A/F) admitted to a diesel engine or a lean-burn gasoline engine is typically in the range of about 20–50 for good fuel economy. As a result the exhaust has a relatively low content of unburned hydrocarbons and carbon monoxide (typically about 200–700 parts per million, ppm, of hydrocarbons, and 500–1500 ppm of carbon monoxide) and a relatively low content of nitrogen oxides such as NO and NO 2 . These nitrogen oxides are referred to collectively as NOx and a typical NOx content in a diesel exhaust is about 50–500 ppm. Due to the oxygen content of the fuel-lean exhaust, unburned HC and CO can be oxidized in a catalytic oxidation converter. But reduction of NOx to nitrogen in the oxygen-containing exhaust has been a challenge. 
     Flow-through traps have been used to momentarily absorb NOx from the fuel-lean exhaust. A reductant, such as a small quantity of diesel fuel is then briefly introduced into the inlet of the lean NOx trap to provide reactive species for the catalytic reduction of the absorbed NOx. The NOx trap may be an extruded cordierite honeycomb structure with hundreds of cells per square inch of cross-section extending from the inlet face to the outlet face of the extrusion. The walls of the cells may be coated with a washcoat of high surface area alumina particles which are impregnated with an absorbent for NOx such as barium carbonate (BaCO 3 ), an oxidation catalyst such as platinum, and a reduction catalyst such as rhodium and platinum. The finely divided platinum in the washcoat promotes oxidation of NO, to NO 2 . NO 2  is absorbed from the lean exhaust by reaction with BaCO 3  to form Ba(NO 3 ) 2  (barium nitrate) with the release of CO 2  into the exhaust. When reductant species, such as CO, HC and/or H 2 , are introduced into the exhaust flowing through the cellular trap, NOx is released from the Ba(NO 3 ) 2  (with regeneration of BaCO 3 ) and reduced to N 2  over the rhodium and platinum particles in the washcoat. 
     Reductant species have been added to the exhaust by briefly injecting more fuel into the cylinders of the engine to a much lower A/F of 12–14. For example, the engine is operated in its normal fuel-lean mode for sixty seconds while NOx is stored in the LNT, and then operated in a fuel rich-mode for three seconds to provide reductant species for NOx removal and reduction. This practice requires more complicated control of engine fuel injection and causes noticeable variation in engine torque. Further, it permits NOx and HC emissions while the trap is being regenerated. 
     Reduction species have also been added directly to the exhaust stream, upstream of the inlet of the LNT, during a trap regeneration period. While this practice minimizes or avoids changes in engine operation, it still interrupts the trapping function of the LNT and permits NOx emissions during each trap regeneration period. Thus, there is a need for an improved practice for regeneration of the LNT. 
     SUMMARY OF THE INVENTION 
     This invention provides a mechanical flow distributor for directing flow of a reductant into only a portion of the inlet face of the LNT. The inlet face of an LNT is normally flat and transverse to gas flow in the exhaust duct or pipe of the engine. The outlet of the distributor lies close to the inlet face to selectively introduce a flow of reductant gas species into only the adjacent LNT cell openings. Thus, normal exhaust flow proceeds through most for the longitudinal cell passages, but the trapped NOx in the passages affected by the distributed reductant flow is removed and reduced to nitrogen while NOx is trapped in the passages not affected by the distributor. 
     The distributor comprises a tubular reductant flow portion terminating in a shaped outlet spanning a predetermined section of the exhaust inlet face of the LNT. The distributor directs reductant gas into the selected cells of the trap for a brief time, sufficient to regenerate the selected cells, while deflecting the lean NOx exhaust into the larger number of remaining trap cells. The shaped outlet of the reductant distributor is then progressively moved over the entire exhaust inlet face of the LNT to continually regenerate other portions of the trap. In a different embodiment, the LNT may be rotated with respect to the shaped outlet of the reductant distributor. 
     In a preferred embodiment, the ceramic LNT is round in its cross-section to exhaust flow. In this embodiment, the outlet of the distributor is preferably shaped to cover a radial or diametrical section of the inlet, and the distributor outlet is rotated co-axially with the LNT to progressively and continually regenerate the trap. For example, at any time the distributor outlet may cover a ⅙ th  or 1/12 th  segment of the LNT inlet for regeneration of that portion while the remainder of the inlets are receiving exhaust gas and trapping NOx. 
     Thus, the engine exhaust stream is shaped to permit positioning of the reductant distributor with its outlet adjacent the inlet of the LNT. The delivery tube of the reductant distributor extends outside the exhaust pipe or duct for receiving reductant gas from a suitable reductant generator or storage vessel. A suitable electromechanical actuator is provided to move, preferably rotate, the distributor outlet progressively over the inlet face of the LNT. Conversely, the LNT may be moved or rotated with respect to the distributor outlet. 
     Suitable, typically small, amounts of lean NOx exhaust may be admitted to the reductant distributor to provide oxygen for thermal oxidation and heating of the reductant and exhaust gases in the cells. In this embodiment, the reductant distributor is preferably fitted with a suitable oxidation catalyst package. The HC oxidation raises the temperature of the reductant-containing gases flowing in the cells of the regenerating part of the LNT and speeds up the NOx reduction. 
     Suitable reductants include diesel fuel, gasoline and other relatively low molecular weight hydrocarbon vapors or gases. As a further example, gaseous reformation or partially oxidized products of hydrocarbon fuels such as a mixture of carbon monoxide, hydrogen and hydrocarbon by-products may be delivered through the reductant distributor to the LNT inlet. 
     Other objects and advantages of the invention will become more apparent from a detailed description of embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a portion of an exhaust system of a lean burn engine, such as a diesel engine. The illustrated portion of the exhaust system includes a lean NOx trap, LNT, and a reductant distributor in accordance with this invention, both located in a section of the exhaust pipe. 
         FIG. 2A  is an oblique view of one embodiment of a reductant distributor. 
         FIG. 2B  is a cross-sectional view taken at direction  2 B— 2 B of  FIG. 1  and showing an instantaneous location of the reductant distributor of  FIG. 2A  opposite a diametric segment of the circular inlet face of the LNT of  FIG. 1 . 
         FIG. 3  is a view like  FIG. 2B  showing another embodiment of a reductant distributor opposite a radial segment of the circular inlet of the LNT of  FIG. 1 . 
         FIG. 4  is a cross-sectional view of a portion of an LNT and still another embodiment of a reductant distributor, both located in a section of the exhaust pipe. In this embodiment the reductant distributor has inlet holes for a fraction of the lean exhaust stream, and an oxidation catalyst for oxidizing a portion of the reductant stream and thereby heating the mixed stream for increasing the temperature of the portion of the LNT being regenerated. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     As stated above, the exhaust from a lean burn engine contains unburned hydrocarbons, carbon monoxide, carbon dioxide, nitrogen oxides (collectively NOx), water, oxygen and nitrogen. Once the engine is warmed-up, the exhaust temperature may be in the range of 150° C. to about 500° C. depending upon engine load and other operating conditions. A portion of the engine exhaust may be re-circulated into the engine air supply. In other engine embodiments, the engine exhaust may first be directed through a turbocharger for increasing engine air intake, or the exhaust may flow directly into the exhaust conduit or pipe carried under the vehicle for exhaust discharge at the rear of the vehicle. Most, if not all, modern diesel engines have turbocharged air intake. Once the exhaust leaves the vicinity of the engine, it may flow through a catalyst-containing oxidation converter to utilize the oxygen in the exhaust to oxidize carbon monoxide to carbon dioxide and to complete the combustion of incompletely burned hydrocarbons. Generally, this oxidation catalyst is used mainly to increase the exhaust temperature for the exhaust after-treatment. This invention is applicable to any such engine and exhaust combination. 
     The NOx containing exhaust from a diesel engine, or other lean burn engine, may then be directed to a lean NOx trap for temporary storage of the oxides of nitrogen. This invention is applicable to the regeneration of the LNT. 
       FIG. 1  illustrates a portion of an exhaust conduit downstream from a diesel engine and any oxidation converter. Exhaust conduit section  10  includes a lean NOx trap-enclosing conduit section  12 , an upstream exhaust conduit section  14  leading to LNT-enclosing section  12 , and a downstream exhaust conduit section  16  leading from LNT-enclosing section  12 . Upstream and downstream exhaust conduit sections  14 ,  16  are usually round in cross-section and sized to carry the exhaust without excessive back-pressure. LNT-enclosing section  12  has a larger diameter circular chamber  18  for retaining the round cylinder LNT  20  between a conical exhaust inlet chamber  22  and a conical exhaust outlet chamber  24 . The respective exhaust conduit sections  14 ,  16 ,  18 ,  22 , and  24  are typically made of a stainless steel material (like much of the exhaust piping system) and joined into a continuous flow passage  12  for the exhaust into and through LNT  20  and downstream for further treatment or discharge to the atmosphere. 
     LNT  20  is often an extruded cordierite honeycomb structure with hundreds of cells (depicted schematically at  30  in  FIGS. 2B and 3 ) per square inch of cross-section extending from the inlet face  26  to the outlet face  28  of the extrusion. The side walls of cells  30  are typically coated with a washcoat of high surface area alumina particles which are impregnated with an absorbent for NOx such as barium carbonate, an oxidation catalyst such as platinum, and a reduction catalyst such as a combination of rhodium and platinum. As summarized above, the finely divided platinum in the washcoat promotes oxidation of NO, to NO 2 . NO 2  is absorbed from the lean exhaust by reaction with BaCO 3  to form Ba(NO 3 ) 2  with the release of CO 2  into the exhaust. When reductant species, such as CO, HC and/or H 2 , are introduced into the exhaust flowing through cellular trap  20  in an oxygen-deficient environment, NOx is released from the Ba(NO 3 ) 2  (with regeneration of BaCO 3 ) and reduced to N 2  over the rhodium and platinum particles in the washcoat. 
     In accordance with this invention, upstream exhaust conduit  14  is curved or otherwise suitably shaped for placement of a reductant gas distributor  32  for delivery of a reductant gas to a portion of the inlet face  26  of LNT  20  and for blocking the exhaust gas, which contains NOx and oxygen from entering the regenerating section. Inlet face  26  is planar and aligned transversely to the exhaust flow. Reductant distributor  32  has a conduit portion  34  leading to a diverging reductant gas distributor outlet  36 . The tubular conduit portion  34  extends through a wall of upstream conduit section  14  and is supported in a sealing bushing  38  which permits rotation of reductant distributor  32  while retaining engine exhaust within the upstream exhaust conduit  14 . A suitable drive connection or self-powered mechanism, shown schematically by block  40 , rotates reductant distributor conduit  34  and attached reductant distributor outlet  36  for progressive delivery of reductant gas into successive portions of cells  30  at inlet face  26  of LNT  20 . A battery powered electric motor may be employed as the reductant distributor rotation mechanism. 
     In an alternative embodiment, means is provided to rotate the LNT  20  within its housing  18  and the reductant distributor  32  is not rotated. The idea is to provide relative motion between the inlet face  26  of the LNT  20  and the reductant distributor outlet  36  for progressive regeneration, portion-by-portion, of the LNT. 
     The reduction gas distributor conduit  34  receives and conducts a stream of reductant gas from a suitable source, not shown, and delivers the reductant material to reductant gas distributor outlet  36  with sufficient pressure to direct the reductant gas flow into a selected segment of cells  30  of the LNT  20  as will be described in connection with  FIG. 2B . The source of the reductant gas may be a gaseous hydrocarbon such as vaporized diesel fuel or gasoline. In another embodiment, such a hydrocarbon fuel may be delivered from on-vehicle storage to a reformer in which the hydrocarbons are converted to more reactive reduction species; for example, a mixture of hydrogen, carbon monoxide and very small HC molecules. In accordance with a preferred embodiment of the invention, a relatively low volume continuous stream of such reformed gas mixture is delivered to the LNT  20  through reductant gas distributor conduit  34  and attached reductant distributor outlet  36 . 
     Reductant gas distributor outlet  36  is shaped and sized to direct the flow of reductant gas into the inlet face  26  openings of a selected segment of cells  30  in LNT  20 . The atmosphere in the selected segment of cells is temporarily changed from an oxidizing environment to a chemically reducing environment. The function of distributor outlet  36  is to disperse or expand and channel the flow of reductant gas into a portion of the cells  30  of the LNT  20  for catalyzed removal of stored NOx and its chemical reduction to nitrogen. In the meantime, oxygen-containing exhaust gas flows around the outside of reductant distributor outlet  36  into the remainder of LNT cells  30  and NOx is absorbed from the exhaust. Thus, throughout the operation of a warmed-up engine, some cells  30  of LNT  20  are receiving reductant gas from distributor outlet  36  for removal and elimination of stored NOx, and the remainder of the cells  30  (usually the majority of the cells) are being used to trap NOx from the exhaust and temporarily store it until the reductant distributor outlet  36  is progressively moved into position for regeneration of these cells. 
     In the exhaust system illustrated in  FIGS. 1 and 2B  the LNT  20  is a round cylinder which is a common design for such a trap. Accordingly, in a preferred embodiment, reductant distributor outlet  36  is shaped and sized to deliver reductant gas to successive diametric sections of inlet face  26  of LNT  20 . Distributor outlet  36  is generally triangular in side elevation view as seen in  FIGS. 1 and 2A . At the apex or narrow end of reductant distributor outlet  36 , is a round inlet  42  connected to the outlet end of reductant distributor conduit portion  34 . The flow axes of reductant conduit  34  and reductant distributor outlet  36  are co-axial with the center of the round LNT  20 . The distributor outlet  36  diverges from its inlet  42  to its outlet end or face  44 . As seen in  FIG. 2B , reductant distributor outlet face  44  is sized and shaped to closely overlie a diametric segment (around diameter  46 ) of inlet face  26  of LNT  20 . Diametrically opposed edges  48  of distributor outlet face  44  lie close to the inner surface of circular chamber  18  enclosing LNT  20 . Sides  50  of distributor outlet  36  are shaped and spaced to cover a diametrical segment around diameter  46 . Perforated diverter strips  54  are each fixed at one end to opposite diverging sides of the outlet  36 . Diverter strips merge (to form a V configuration) and are fixed together at the other end in a joint at the flow axis of the distributor outlet  36 . Diverter strips  54  direct reductant gas flow in an expanding or diverging path to enter enclosed LNT cells  30  along the full diameter of the LNT inlet face segment being treated. 
     In its operation, reductant gas distributor  32  selectively directs a flow of reductant gas material into the LNT cell  30  openings covered by the outlet face  44 . Engine exhaust gas flows around the distributor conduit  34  and distributor outlet  36  into the larger number of LNT cells left open by reductant gas distributor  32 . 
     In  FIG. 2A , distributor outlet  36  is seen covering a diametrical segment of LNT inlet face  26  spaced about inlet face diameter  46 . After a period of, for example, a few seconds during which the reductant flow brings about the removal of NOx from the covered segment (and its reduction to N 2 ) of the LNT, drive  40  rotates distributor  32  so that distributor outlet  36  covers a different diametrical segment about diameter  52  (for example). After each brief regeneration period the reductant distributor  32  is moved to successively regenerate each section of the LNT  20 . In this example, six such regeneration periods are required to complete one regeneration cycle of the entire LNT. While about one-sixth of the LNT is being stripped of stored NOx, the remaining five-sixths of the inlet face area is receiving exhaust and continuing to trap NOx. 
     It is now apparent that the distributor outlet portion  36  of the reductant gas distributor  32  is shaped to treat a predetermined, relatively small portion of the inlet face  26  of LNT  20 . In general it is preferred, but not required, that the LNT be round in cross-section at its inlet so that the distributor  32  can be rotated over a circular area. However, other LNT inlet face profiles can be accommodated by appropriated sizing, shaping, and movement of the distributor outlet portion  36 . 
     Since this invention permits more or less continual regeneration of an LNT, the size of the reductant distributor outlet  36  may be smaller to more fully utilize the cross-sectional area of the trap.  FIG. 3  illustrates a second embodiment of a reductant distributor  132  with a distributor outlet  136  shaped to treat only a radial segment (around radius  146 ) of inlet face  26  of LNT  20 . In this embodiment, reductant gas entering inlet  142  of distributor outlet regenerates only about one-twelfth of the LNT at one time. 
     The relative motion between the reductant gas distributor and the LNT can progress stepwise, as described, or continuously at a rate determined to fully regenerate the LNT upon one full rotation of the distributor or the LNT. 
       FIG. 4  illustrates another embodiment of the invention in which the reductant distributor admits a small portion of the lean exhaust steam for the purpose of oxidizing a portion of the reductant stream and heating it to accelerate regeneration of the LNT. In  FIG. 4 , reductant distributor  232  is formed with small exhaust gas inlet holes  256  (two shown in  FIG. 4 ) in the reductant distributor outlet  236  near inlet  242  where it is attached to reductant distributor conduit  234 . Such exhaust gas inlet holes  256  permit a small amount of oxygen-containing gas to enter the reductant distributor outlet  236 . As indicated by the arrows in upstream exhaust conduit  14  and conical exhaust inlet chamber  22  most of the lean exhaust stream flows around the reductant distributor  232  and enters portions of the LNT  20  not covered by reductant distributor outlet  236 . However, a small portion of the lean exhaust enters exhaust gas inlet holes  256  and mixes with the reductant stream flowing through reductant distributor conduit  234  and inlet  242  of the distributor outlet  236 . 
     In this embodiment, reductant distributor outlet  236  contains an oxidation catalyst such as small particles of platinum or palladium dispersed on beads of high surface area alumina. Reductant distributor outlet comprises an upstream screen  260  and an axially displaced downstream screen  262 . The space between screens  260  and  262  is filled with oxidation catalyst beads  264 . Thus, beads  264  make a flow-through oxidation catalyst bed. 
     The oxygen containing exhaust mixes with reductant species from reductant distributor conduit  234  in reductant distributor outlet  236  and flows through beads  264 . The amount of admitted oxygen containing lean exhaust oxidizes a portion of the reductant species in the mixture to heat the stream exiting reductant distributor outlet face  244  and entering the portion of the LNT inlet face  26  covered by reductant distributor outlet  236 . The flow of reductant and admitted lean exhaust is managed for regeneration of the cells  30  of the LNT  20  then being treated. Thus, the reducing atmosphere is heated within reductant distributor outlet  236  for faster NOx removal from the affected cells of the LNT  20  and reduction to nitrogen (regeneration). The heated reductant stream then enters those longitudinal cells  30  ( FIGS. 2B and 3 ) of LNT  20  covered by reductant distributor outlet  236 . Progressive or continuous rotation of reductant distributor  232  is employed to periodically regenerate the whole LNT  20 . 
     Thus, in this reductant distributor configuration, it is preferred that a suitably shaped oxidation catalyst package, such as a bed of beads  264 , is installed inside the reductant distributor outlet  236 . The oxidation catalyst will promote the exothermic reaction within the reductant distributor outlet  236  between the oxygen and reductants flowing to the LNT  20 . In some embodiments of the reductant distributor it may be preferred to use the oxidation catalyst in a different form in the reductant distributor outlet. It may be preferred to deposit the oxidation catalyst material as a washcoat on a metal or ceramic monolith support. In this embodiment, the catalyst bearing monolith would be suitably positioned and fixed in the reductant distributor outlet  236  in a manner like screens  260  and  262  and beads  264  are positioned and fixed. 
     It is recognized that the overall exhaust system may also be designed to contain an oxidation reactor downstream of the LNT and the reductants gas distributor to complete combustion of residual reducing species. 
     While the invention has been described in terms of a few preferred embodiments, it is apparent that other embodiments within the scope of the invention can readily be adapted by those skilled in the art.