Patent Document

FIELD OF THE INVENTION 
     The present invention relates to emissions control systems for vehicles, and more particularly to emissions control systems that reduce oxides of nitrogen in emissions. 
     BACKGROUND OF THE INVENTION 
     Engine operation involves combustion that generates exhaust gas. During combustion, an air and fuel (air/fuel) mixture is combusted inside a cylinder to drive a piston. The piston rotatably drives a crankshaft that ultimately rotates one or more camshafts. Exhaust gas is created from combustion and is released from the cylinders into an exhaust system. The amount of exhaust gas released is regulated by the opening and/or closing positions of an exhaust valve that is mechanically actuated by a cam lobe coupled to the camshaft. The exhaust gas may contain residuals such as, oxides of nitrogen (NOx) and carbon monoxide (CO). 
     Retaining exhaust gas inside the cylinder during the exhaust stroke, also known as exhaust gas retention, burns increased levels of NOx during the following combustion stroke and may decrease levels of emissions exiting the engine. Specifically, retaining exhaust gases in the combustion chamber of the cylinder dilutes the air/fuel mixture and slows the burn rate. The reduced burn rate results in increased combustion chamber temperatures for a longer period of time and burns greater amounts of NOx to reduce emissions. 
     Exhaust gas retention can be accomplished by adjusting the rotational position of the exhaust camshaft to vary the timing of the exhaust valve. The valve timing determines the amount of exhaust that remains in the cylinder during the exhaust stroke. Levels of NOx retained at various speeds and loads are predetermined and programmed in a static reference table. 
     Although design differences and component wear can effect engine operation, exhaust gas retention is typically limited to the static reference table. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a control system for adjusting levels of emissions exiting an engine with a camshaft that is associated with an exhaust valve and a cam phaser that interfaces with the camshaft. The control system includes a NOx sensor that generates a NOx signal in response to oxides of nitrogen (NOx) in an exhaust gas and a control module that communicates with the cam phaser. The control module receives the NOx signal, and calculates a NOx level of the exhaust gas based on the NOx signal. The control module compares the NOx level to a predetermined threshold range and adjusts the cam phaser to achieve a rotational position that releases a desired level of NOx from the engine when the NOx level exceeds the predetermined threshold range. The control module stores a rotational position value based on the rotational position of the cam phaser in a storage device when the NOx level is within the predetermined threshold range. The rotational position of the cam phaser controls an actuation time when the camshaft opens the exhaust valve during rotation of the camshaft. 
     In one feature, the exhaust valve position determines an amount of the exhaust gas that exits the engine. 
     In another feature, the predetermined threshold range is defined as having an upper NOx level value and a lower NOx level value. 
     In yet another feature, the storage device includes a two-dimensional reference table that is indexed by a range of predetermined speed (RPM) values and a range of predetermined mass air flow (MAF) values. 
     In still another feature, the control module stores a rotational position value based on the rotational position according to a corresponding speed value and a corresponding load value included in the reference table. 
     In yet another feature, the control module adjusts a rotational position of the cam phaser based on a rotational position value included in the reference table when the engine operates at a corresponding speed and a corresponding load included in the reference table. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram of an engine control system providing an emissions control system using a NOx sensor according to the present invention; and 
         FIG. 2  is a flow chart illustrating steps executed by an emissions control system according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     Referring now to  FIG. 1 , an engine system  10  is schematically illustrated. The engine system  10  includes an engine  12  that combusts an air and fuel (air/fuel) mixture to produce drive torque. Air is drawn into an intake manifold  14  through a throttle  16 . The throttle  16  regulates mass air flow into the intake manifold  14 . Air within the intake manifold  14  is delivered into cylinders  18  through  14  an intake valve (not shown). Although three cylinders  18  are illustrated, it can be appreciated that the emissions control system of the present invention can be implemented in engines having a plurality of cylinders  18  including, but not limited to, 2, 3, 4, 5, 6, 8, 10 and 12 cylinders. 
     A fuel injector (not shown) injects fuel which is combined with the air as it is drawn into the cylinder  18  through an intake port (not shown). The fuel injector can be an injector associated with an electronic or mechanical fuel injection system (not shown), or another system for mixing fuel with intake air. The fuel injector is controlled to deliver a desired air/fuel ratio within each cylinder  18 . Typically, one unit of fuel is delivered for every 14.7 units of air delivered into the cylinder. 
     An intake valve  20  selectively opens and closes to enable the air/fuel mixture to enter the cylinder  18 . The intake valve position is regulated by an intake camshaft  22 . A piston (not shown) compresses the air/fuel mixture within the cylinder  18 . A spark plug (not shown) initiates combustion of the air/fuel mixture and drives the piston in the cylinder  18 . The piston drives a crankshaft  24  to produce drive torque. The crankshaft  24  rotatably drives camshafts using a timing chain (not shown) to regulate the timing of the intake and exhaust valves  20 ,  26 . Although a single intake camshaft and a single exhaust camshaft are shown  20 ,  28 , it can be anticipated that a single camshaft or dual intake camshafts and dual exhaust camshafts may be used. 
     Exhaust gas is produced inside the cylinder  18  as a result of the combustion process. The exhaust gas is forced out an exhaust port (not shown) into an exhaust manifold  29  when an exhaust valve  26  is in an open position. The exhaust gas may be treated by an exhaust treatment system (not shown) prior to exiting into the atmosphere. Although single intake and exhaust valves  20 ,  26  are illustrated, it can be appreciated that the engine  12  can include multiple intake and exhaust valves  20 ,  26  per cylinder  18 . 
     Intake and exhaust cam phasers  30 ,  32 , respectively, adjust the rotational position of the intake and exhaust camshafts  22 ,  28 , respectively. More specifically, the rotational position of the intake and exhaust camshafts  22 ,  28  can be retarded and/or advanced with respect to each other or with respect to a location of the piston within the cylinder  18  or the rotational position of the crankshaft  24 . In this manner, the timing and/or lift of the intake and exhaust valves  20 ,  26  can be varied with respect to each other or with respect to a location of the piston within the cylinder  18 . By varying the lift position of the exhaust valve  26 , the amount of exhaust retained in the cylinder  18  can be adjusted. 
     The engine system  10  further includes a NOx sensor  34  and a control module  36 . The NOx sensor  34  is responsive to exhaust gas and outputs a NOx signal (NOx SIGNAL ) indicating levels of NOx exiting the engine  12 . The NOx sensor  34  can sense exhaust gas chemically, optically, or using another method. 
     The control module  36  receives NOx SIGNAL  and adjusts levels of emissions exiting the engine  12  based on a predetermined threshold range. The threshold range can be defined as having an upper NOx level value and a lower NOx level value. Prior to adjusting the exhaust cam phaser  32 , the control module  36  determines the level of NOx exiting the engine  12  based on NOx SIGNAL  and compares the level of NOx exiting the engine  12  to the predetermined threshold range (NOx THR ). NOx THR  is defined as having an upper NOx level value and a lower NOx level value. When the NOx level exiting the engine  12  is not within NOx THR , the control module  36  outputs a cam phaser control signal that rotatably adjusts the exhaust cam phaser  32 . The exhaust cam phaser  32  receives the cam phaser control signal and rotatably adjusts the exhaust cam phaser position (θ EXHAUST     —     CAM ). The position of the cam phaser  32  advances and/or retards the actuation time at which the exhaust camshaft  28  opens and/or closes the exhaust valve  26 . The control module  36  repeats the operation described above until the level of NOx exiting the engine  12  is within NOx THR . 
     The control module  36  can store θ EXHAUST     —     CAM  in a two-dimensional reference table. The reference table can be indexed by a predetermined range of speed (RPM) values and a predetermine range of mass air flow intake (MAF) values. When the NOx exiting the engine  12  is within NOX THR , the control module  36  stores θ EXHAUST     —     CAM  according to a respected RPM value and respected MAF value. The control module  36  can refer to the reference table in future driving scenarios and can adjust the exhaust camshaft  28  based on the stored θ EXHAUST     —     CAM  when similar a operating condition (i.e. a similar speed and a similar load) is encountered. 
     For example, the control module  36  outputs Cam ADV  to advance the exhaust camshaft  28  when the level of NOx exiting the engine  12  exceeds NOx THR . Advancing the exhaust cam phaser  32  during the exhaust stroke advances the actuation time when exhaust camshaft closes the exhaust valve  26 . Advancing the closing position of the exhaust valve  26  prevents an amount of exhaust gas from escaping the cylinder  18 . The retained exhaust gas dilutes the air/fuel mixture and lowers the combustion temperature below a point at which nitrogen combines with oxygen to form NOx. As a result, the level of NOx exiting the engine  12  can be reduced. 
     The control module  36  can further determine whether θ EXHAUST     —     CAM  was adjusted properly. Specifically, the control module  36  measures an initial level of NOx exiting the engine  12  prior to adjusting the exhaust cam phaser  32  (NOx PRE ). After adjusting the exhaust cam phaser  32 , the control module  36  remeasures the level of NOx after adjusting the exhaust cam phaser  32  (NOx POST ). When NOx POST  exceeds NOx PRE , the control module  36  assumes θ EXHAUST     —     CAM  was rotated in the wrong direction. During the subsequent exhaust stroke, the control module  36  adjusts the rotation of exhaust cam phaser  32  in the opposite direction. 
     Referring now to  FIG. 2 , a flowchart illustrates the steps executed by the control system according to the present invention. In step  200 , control determines the level of NOx exiting the engine  12  prior to adjusting the exhaust cam phaser  32  (NOx PRE ) based on NOx SIGNAL . In step  203 , control compares NOx PRE  to NOx THR . When NOx PRE  exceeds NOx THR , control advances θ EXHAUST     —     CAM  based on Cam ADV  and measures a second level of NOx (NOx POST ) subsequent to adjusting the exhaust cam phaser  32  in step  204 . Otherwise, control returns to step  200 . Although the flowchart describes initially advancing the exhaust cam phaser  32 , it can be appreciated that the invention can initially retard the exhaust cam phaser  32 . 
     In step  206 , control compares NOx POST  to NOx PRE  and determines whether advancing the exhaust cam phaser  32  causes the level of NOx exiting the engine  12  to decrease. If NOx POST  is less than NOx PRE , then control compares NOx POST  to NOx THR  in step  208 . Otherwise, control proceeds to step  210 . In step  208 , control determines whether NOx POST  is within NOx THR . When NOx POST  is within NOx THR , control stores θ EXHAUST     —     CAM  and control returns to step  200 . Otherwise, control returns to step  204  and continues advancing the exhaust cam phaser  32 . 
     In step  210 , control retards the exhaust cam phaser  32  and remeasures NOx POST . In step  212 , control compares NOx POST  to NOx THR . When NOx POST  is within NOx THR , control stores θ EXHAUST     —     CAM , in step  209  and control returns to step  200 . Otherwise, control returns to step  210 , and continues adjusting the exhaust cam phaser  32 . 
     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.

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