Abstract:
An exhaust after-treatment system for an internal combustion engine includes a lean NOx catalyst having an exhaust stream from the internal combustion engine flowing therethrough. A NOx absorber catalyst is downstream of the lean NOx catalyst. The NOx absorber is selectively regenerated to increase a NOx reduction efficiency of the exhaust after-treatment system.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to internal combustion engines, and more particularly to an optimized NOx reduction exhaust system. 
       BACKGROUND OF THE INVENTION 
       [0002]    Internal combustion engines generate drive torque by combusting an air and fuel mixture within cylinders. Exhaust that is generated via the combustion process is exhausted from the cylinders and is treated in an after-treatment system. During the combustion process, fuel is oxidized and hydrogen (H) and carbon (C) combine with air. Various chemical compounds are formed including carbon dioxide (CO 2 ), water (H 2 O), carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons (HC), sulfur oxides (SOx), and other compounds. 
         [0003]    The after-treatment systems traditionally include a catalytic converter that reduces exhaust emissions by chemically converting the exhaust gas into carbon dioxide (CO 2 ) nitrogen (N), and water (H 2 O). In some cases, a lean NOx catalyst is implemented. Lean NOx technology, also known as HC selective catalytic reduction (SCR) has various formulations (e.g., platinum/alumina, copper and substituted zeolite. Platinum on alumina (Pt/Al 2 O 3 ) functions at law temperatures has higher peak conversion of approximately 40% at 225° C., but has a very narrow temperature window of operation (e.g., between 180-280° C.). As a result, this formulation is not very useful by itself. Another disadvantage of platinum catalysts has been their SOx oxidation activity and if s susceptibility to deactivation by sulfur. 
         [0004]    NOx absorbers have also been developed based on acid-base wash-coat chemistry. The NOx is absorbed and is stored in the NOx absorber catalyst wash-coat during lean operating conditions (i.e., higher than stoichiometric air to fuel ratio). The NOx is released and is catalytically converted to nitrogen during rich operating conditions (i.e., lower than stoichiometric air to fuel ratio). Barium-based NOx absorbers have high conversion efficiency but are only active at increased temperatures (e.g., greater than approximately 250° C.). Also, NOx absorbers require periodic desulfation. 
       SUMMARY OF THE INVENTION 
       [0005]    Accordingly, the present invention provides an exhaust after-treatment system that overcomes the deficiencies of the above-described after-treatment technologies. The exhaust after-treatment system includes a lean NOx catalyst having an exhaust stream from said internal combustion engine flowing therethrough. A NOx absorber catalyst is downstream of the lean NOx catalyst. The NOx absorber is selectively regenerated to increase the NOx reduction efficiency of the exhaust after-treatment system. 
         [0006]    In another feature, the NOx absorber catalyst is regenerated when the NOx reduction demand is greater than a high threshold. 
         [0007]    In another feature, the lean NOx catalyst alone reduces a NOx content of the exhaust stream when a NOx reduction demand is less than a low threshold. 
         [0008]    In still other features, hydrocarbon (HC) is introduced into the exhaust stream when a NOx reduction demand is greater than a first threshold and is less than a second threshold. The HC is introduced via at least one of spark advance, post-combustion in-cylinder fuel injection and in-exhaust injection downstream of the internal combustion engine. 
         [0009]    In yet another feature, the exhaust after-treatment system further includes a hydrocarbon (HC) dosing unit disposed upstream of one of the lean NOx catalyst and the NOx absorber catalyst. 
         [0010]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0012]      FIG. 1  is a functional block diagram of an engine system including an optimized NOx reduction exhaust system in accordance with the present invention; and 
           [0013]      FIG. 2  is a flowchart illustrating exemplary steps executed using an optimized NOx reduction control of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the term module refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
         [0015]    Referring now to  FIG. 1 , an engine system  10  includes an internal combustion engine  12  and an optimized NOx exhaust system  14  in accordance with the present invention. The engine  12  generates drive torque that is used to propel a vehicle within which the engine system  10  is implemented, and/or drive accessory loads including, but not limited to, an alternator and a fluid pump (not shown). Air is drawn into an intake manifold  16  through a throttle  18 . The air is distributed to cylinders (not shown) of the engine  12  and is mixed with fuel to form a combustion mixture. The combustion mixture is ignited within the cylinder to reciprocally drive a piston (not shown). The combustion processes generates exhaust gas that exits the engine  12  through an exhaust manifold  20  and that is treated in the optimized NOx exhaust system  14 . 
         [0016]    A control module  22  regulates operation of the engine system  10  based on various engine system operating conditions. A manifold absolute pressure (MAP) sensor  24  is responsive to the vacuum pressure within the intake manifold  16  and generates a MAP signal based thereon. An engine RPM sensor  26  is responsive to an engine RPM and generates a signal based thereon. An exhaust temperature sensor  28  is disposed downstream of the exhaust manifold  20  and is responsive to the exhaust temperature (T EXH ) and generates a signal based thereon. 
         [0017]    The optimized NOx exhaust system  14  includes a lean NOx catalyst  30  disposed upstream of a NOx absorber catalyst  32 . An HC dosing unit  34  is disposed upstream of the lean NOx catalyst  30 . The HC dosing unit  34  selectively injects HCs into the exhaust stream, which is used for lean NOx catalyst  30  and NOx absorber catalyst  32  regeneration and desulfation. It is also anticipated that post-combustion fuel injection can be implemented to increase the HC content of the exhaust stream, whereby fuel is injected into the cylinder to be exhausted from the cylinder with the exhaust. Further, advanced combustion (i.e. HCCl) includes relatively higher engine out HC levels. This increased HC level is effectively utilized to reduce NOx over the lean NOx catalyst  30  while oxidizing the remaining HCs to release thermal energy. 
         [0018]    The combination of the lean NOx catalyst  30  and the NOx absorber catalyst  32  overcomes the shortcomings of the individual components. The so-configured optimized NOx exhaust system  14  enables the lean NOx catalyst  30  and the NOx absorber catalyst  32  to each be half the size if each were to be used individually. Because 75% of the NOx reduction occurs in the first half of the catalyst, for both the lean NOx catalyst  30  and the NOx absorber catalyst  32 , reducing the size of each by half only reduces the NOx conversion efficiency by 25%. 
         [0019]    Both active and passive lean approaches can be implemented based on the NOx conversion efficiency requirements and engine out HCs. During the active approach, HC dosing in the exhaust and/or in the cylinder (i.e., post combustion fuel injection) is enabled. Further, the optimized NOx exhaust system enables tunable NOx conversion efficiency. For example, if only 25-30% reduction is required under certain operating conditions, only the lean NOx catalyst  30  is used, while the NOx absorber catalyst  32  stays inactive. If higher efficiency is desired, the NOx absorber catalyst  32  is periodically regenerated to store and reduce NOx to nitrogen, as well. As a result, fuel consumption is reduced. Further, the NOx absorber catalyst also acts as a clean-up catalyst for NO if so required. 
         [0020]    During normal operation, if the engine out HC content is high and NOx reduction demand is low, no active control is required. The optimized NOx exhaust system  14  performs the required NOx reduction. If the NOx reduction demand increases, the HC content is increased to correspondingly increase the carbon to NOx ratio, thereby increasing the lean NOx reduction efficiency. If further NOx reduction is desired, the NOx absorber catalyst  32  can be periodically regenerated to store and convert NOx to N 2 . In this manner, a wide range of NOx conversion is achieved, which would otherwise not be possible with the lean NOx catalyst  30  or the NOx absorber catalyst  32  alone. Also, fuel consumption can be reduced because the HC rich exhaust for the NOx absorber catalyst regeneration is not required all of the time. Further, because the NOx absorber catalyst  32  is not used all of the time, its life is prolonged. 
         [0021]    Referring now to  FIG. 2 , exemplary steps executed by the optimized NOx exhaust control will be described in detail. In step  200 , control determines whether the NOx reduction demand is less than a high threshold (THR HI ) (e.g., 75%). If the NOx reduction demand is not less than THR HI , control regenerates the NOx absorber in step  202  and control ends. In this manner, both the NOx absorber catalyst  32  and the lean NOx catalyst  30  are implemented to achieve the higher conversion efficiency. If the NOx reduction demand is not less than THR HI , control continues in step  204 . 
         [0022]    In step  204 , control determines whether the NOx reduction demand is less than a low threshold (THR LO ) (e.g., 30%). If the NOx reduction demand is not less than THR LO , control increases the HC content of the exhaust In step  206  and control ends. In this manner, the conversion efficiency can be increased without regenerating the NOX absorber catalyst  32 . If the NOx reduction demand is not less than THR LO , control uses the lean NOx catalyst only in step  208  and control ends. 
         [0023]    Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.