Abstract:
A method of controlling a dual path NOx adsorber system. The amount of exhaust into the two paths of the system is controlled by a diverter valve at the beginning of each path. The valves are operated such that most of the time, the adsorbers are simultaneously in adsorption mode. The adsorbers alternate with each other to briefly enter a regeneration mode, during which they receive a very small amount of the exhaust flow and also receive supplemental fuel injection.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    This invention relates to exhaust gas emission control for internal combustion engines, and more particularly to controlling nitrogen oxides using nitrogen oxide adsorbers.  
         BACKGROUND OF THE INVENTION  
         [0002]    Today&#39;s conventional diesel engines produce nitrogen oxides (NO x ), which play a role in forming photochemical smog. Yet diesel engines are so durable, reliable, and efficient, it is important to keep them as viable options for transportation.  
           [0003]    Increasingly strict regulations have prompted research for new ways of controlling the polluting emissions from diesel engines without compromising fuel economy. In the United States, Environmental Protection Agency rules require cleaner diesel fuel (lower sulfur content) and more stringent engine standards (reducing particulate matter and nitrogen oxide emissions).  
           [0004]    Researchers have demonstrated that nitrogen oxide emissions can be reduced by exhaust recirculation in both gasoline and diesel engines. However, only a limited amount of exhaust can be recirculated without reducing power output and fuel economy. Recirculated soot particles may also cause wear in diesel engines. Other methods for NOx reduction that are being studied are fuel-water emulsion, selective catalytic reduction with ammonia or urea, lean NOx catalysts, and NOx adsorbers.  
           [0005]    NOx adsorbers (NOx traps) are a promising development as results show that NOx adsorber systems are less constrained by operational temperatures than lean NOx catalysts. NOx traps adsorb and store NOx under lean conditions. A typical approach is to speed up the conversion of nitric oxide (NO) to nitrogen dioxide (NO 2 ) using an oxidation catalyst so that NO 2  can be readily stored as nitrate on alkaline earth oxides. A brief return to stoichiometric or rich operation for one or two seconds is enough to desorb the stored NOx and provide the conditions of a conventional three-way catalyst mounted downstream to destroy NOx.  
           [0006]    One design for using NOx adsorber technology is referred to as a “dual leg” or “dual path” design. In these systems, the exhaust path splits into two paths, with a NOx adsorber on each new path. Typically, one path receives most of the exhaust flow while the other path is in regeneration mode. After regeneration, that path begins to receive most of the exhaust flow while the other path regenerates. The paths continually switch modes in this manner.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 illustrates a dual path NOx adsorber system in accordance with the invention.  
         [0008]    [0008]FIG. 2 illustrates a method of controlling a dual path NOx adsorber system in accordance with the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]    [0009]FIG. 1 illustrates a dual path NOx adsorber system  10  in accordance with the invention. Engine  11  is assumed to be a diesel internal combustion engine, but the invention could be modified for other types of engines that use NOx adsorption for treating exhaust.  
         [0010]    Exhaust is exhausted from engine  11  into dual path NOx adsorber system  10 , which is described below. In the example of this description, system  10  is immediately downstream of engine  11 . An oxidation catalyst (OC)  12  and a carbon soot filter (CSF)  13  are downstream of the NOx adsorber system  10 .  
         [0011]    In other embodiments of the invention, the arrangement of system  10  relative to any other exhaust treatment device(s) could be varied. Thus, oxidation catalyst  12  and/or carbon soot filter  13  could be upstream of system  10  rather than downstream, and other treatment devices could be placed upstream or downstream.  
         [0012]    System  10  has two lean NOx traps (LNTs)  14 , one on each path of system  10 . NOx adsorber technology uses a three step process for the reduction of NOx. The first step is for the NO and excess O 2  in the exhaust gas to be chemically converted to NO2 over a platinum-based catalyst during lean engine operation. The second step is storing or adsorbing the NO2 as a nitrate on an site within a catalyst substrate. Before NO2 saturation is reached, the third step is to supply enough reductant (CO, HC, H2, etc) in the exhaust to maintain a rich operation long enough to chemically react the stored NO2 to produce nitrogen, water, and carbon dioxide. Various NOx adsorption devices could be used, with the common feature being NO2 saturation and the need for periodic regeneration (desorption).  
         [0013]    Typically, each adsorber  14  has substantially equal adsorption capacity. As explained below, during engine operation, adsorbers  14  may be in either “adsorption model” or “regeneration mode”. In adsorption mode, each adsorber  14  receives exhaust appropriate for its relative adsorption capacity, typically one-half.  
         [0014]    In adsorption mode, approximately one-half of the exhaust from engine  11  is directed to a first path of system  10  and the other half to the second path. When an adsorber  14  nears NO2 saturation, it enters regeneration mode.  
         [0015]    Various means may be used to determine when adsorbers  14  require regeneration (by being saturated or by having reached a predetermined NOx conversion efficiency). In the example of FIG. 1, NOx sensor  19   a  is placed upstream of adsorbers  14  (and upstream of valves  15 ) and NOx sensors  19   b  are placed immediately downstream of adsorbers  14 . Sensors  19   a  and  19   b  sense the NOx content of the exhaust and deliver a signal to control unit  18 . Control unit  18  processes the sensor signals to determine NOx conversion efficiency. This parameter is used to determine when regeneration is required. Control unit  18  may be part of a more comprehensive engine control unit.  
         [0016]    NOx sensors  19   a  and  19   b  allow NOx conversion efficiency (CE) to be calculated as follows:  
           CE =upstream NOx−downstream NOx×100/upstream NOx  
         [0017]    This calculation is performed in control unit  18 , and can be used to maintain the NOx conversion efficiency at a predetermined rate, such as 95%. The desired efficiency determines the regeneration frequency. The adsorbers are regenerated prior to being saturated, so as to avoid NOx breakthrough.  
         [0018]    Regeneration is accomplished by restricting the flow down the path being regenerated to approximately five percent or less of the total exhaust flow from engine  11 . During regeneration, the remainder of the flow is directed to the second path. Diverter valves  15  are used for this purpose.  
         [0019]    During regeneration, the regenerating path receives supplemental diesel fuel injected into the exhaust stream upstream from adsorber  14 . One of the injectors  16  is used for this purpose. The fuel injection is sufficient so as to achieve an air-fuel ratio of 0.6&lt;λ&lt;0.8 for a period of time less than ten seconds. Other regeneration methods could be used, alternatively or in addition to supplemental fuel injection, such as intake throttling and/or exhaust gas recirculation (EGR).  
         [0020]    At completion of the regeneration period for the first path, valves  15  are activated so as to again direct the exhaust in equal amounts to both paths of system  10 . Thus, both adsorbers  14  are again in adsorption mode. Shortly thereafter, the second path is regenerated using the same scheme as described above for regeneration of the first path. That is, the exhaust flow is reduced and supplemental fuel is injected. At completion of regeneration of the second path, the valves  15  are activated to once again direct the exhaust in equal amounts to both paths. System  10  remains in this state until the next regeneration cycle occurs.  
         [0021]    [0021]FIG. 2 further illustrates the method of controlling system  10 . For purposes of example in FIG. 2, it is assumed that adsorbers  14  are to be regenerated approximately on a cycle of less than 5 minutes. Normally, a lean-to-rich exhaust condition is 30/3, which means that that NOx adsorbers  14  run 3 seconds rich for every 30 seconds of lean operation.  
         [0022]    As illustrated, in Step  21 , both paths are in adsorption mode for a period of less than 5 minutes. Each path receives an appropriate amount of exhaust, which is typically one-half the exhaust where adsorbers  14  are of the same capacity. Then, in Step  22 , a first path is placed regeneration mode, receiving approximately 5% or less of the exhaust. The remainder of the exhaust is directed to the second path while the first path is in regeneration mode. As stated above, the regeneration mode is of brief duration, for example, 10 seconds. In Step  23 , both paths are again placed in adsorption mode, each receiving its fair share of exhaust. In the example of FIG. 2, this second adsorption mode is very brief. In Step  24 , the second path is placed in regeneration mode, and receives approximately 5% or less of the exhaust while in that mode.  
         [0023]    It should be understood that the durations of the adsorption and regeneration modes may vary and that various timing and sensing schemes could be used to control the duration of each mode. For example, experimentation may reveal that a particular frequency of regeneration is optimum. It is also conceivable that Step  23  could be omitted, such that the second path is regenerated immediately after the first path and such that the two paths are simultaneously in adsorption mode only during Step  21 .  
         [0024]    The method described above can be implemented with a control unit  18 , which contains a processor and appropriate memory. Control unit  18  is programmed to generate appropriate control signals to diverter valves  15  and injectors  16 . It may receive signals from NOx sensors  19   a  and  19   b  to determine when regeneration is required, or as discussed below, may include timing means to determine when regeneration is to occur.  
         [0025]    Alternatively, sensors  19   a  and  19   b  may determine the need for regeneration by sensing NOx “breakthrough” past adsorbers  14 . If regeneration fails to revive the adsorber  14 , a desulfurization process may be required. This occurs when sulfur from fuel or oil occupies the catalyst sites in the adsorber and denies them to NOx. Desulfurization may be accomplished thermally by heating the adsorber.  
         [0026]    As a result of the above-described regeneration method both adsorbers  14  are in use during the adsorption period. They are adsorbing in unison, and regenerating successively for brief alternating periods. Both adsorbers are together in adsorption mode substantially longer than the time they are in regeneration mode. For example, if the total regeneration time for both adsorbers  14  was  5  seconds per cycle, they would be in adsorption mode and receiving equal amounts of exhaust for much longer time per cycle.  
         [0027]    This increases the overall capacity of system  10  to adsorb NOx. In effect, the method reduces the space velocity of system  10  as compared to other dual path adsorber systems, where “space velocity” is defined as the maximum engine flow divided by the total NOx adsorber catalyst volume. The residence time for NOx adsorption is increased, and the overall NOx reduction is improved. Experimentation has been successful with an adsorber-to-engine-displacement ratio of 2.9:1.  
         [0028]    System  10  provides satisfactory NOx reduction without the use of exhaust gas recirculation. Thus, the durability of modern diesel engines is not compromised by recirculated exhaust. In addition, the reduced cost of the precious metal required to construct the adsorbers makes the above-described system even more attractive.  
       OTHER EMBODIMENTS  
       [0029]    Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.