Abstract:
A method for determining deterioration of an exhaust gas sensor coupled downstream of an emission control device by monitoring the sensor&#39;s response to a change in an air-fuel ratio is presented. In particular, the sensor is monitored for a predetermined time period following a switch from lean to rich operation and a ratio of a maximum and minimum value is determined. The ratio is then compared to a threshold value to evaluate sensor performance. This method achieves improved emission control and fuel economy.

Description:
BACKGROUND OF INVENTION 
     The present invention relates to a system and a method for monitoring an engine emission control system, and in particular to monitoring a NOx sensor coupled downstream of an emission control device. 
     Internal combustion engines are typically coupled to an emission control device known as a three-way catalytic converter (TWC) designed to reduce combustion by-products such as carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx). Engines can operate at air-fuel mixture ratios lean of stoichiometry, thus improving fuel economy. For lean engine operation, an additional three-way catalyst commonly referred to as a Lean NOx Trap (LNT), is usually coupled downstream of an upstream catalytic converter. The LNT stores exhaust components, such as oxygen and NOx, during lean operation. Continued lean operation eventually saturates the LNT with the selected exhaust gas constituents. After the LNT is filled to a predetermined capacity, stored exhaust gas constituents are typically reduced and released (purged) by switching to rich or stoichiometric operation, i.e., by increasing the ratio of fuel to air and thereby increasing the amount of reductant such as hydrocarbon (HC) present in the exhaust gas mixture entering the LNT. Once the purge is completed, lean operation resumes again. 
     One way of determining when to purge the LNT is by installing a sensor capable of measuring an amount of NOx in the exhaust gas exiting the LNT. Typically, the sensor is monitored to determine when the amount of tailpipe NOx emissions in grams/mile exceeds a predetermined threshold in order to discontinue lean operation. Over time, the performance of the NOx sensor can deteriorate due to such causes as contamination or electrical degradation. This can result in an incorrect determination of when to end lean operation, and may result in lean operation being too long or too short, thus degrading emission control or fuel economy. It is therefore desirable to monitor the performance of the emission control system, and in particular to detect the degradation of the NOx sensor. 
     One method of NOx sensor monitoring is described in U.S. Pat. No. 5,426,934, wherein a NOx catalyst is coupled to an upstream (post-catalyst) and a downstream (pre-catalyst) NOx sensor. Only the upstream (pre-catalyst) NOx sensor is monitored for deterioration. The method includes comparing the ratio of the NOx sensor output during lean operation (NO xlean ) and the NOx sensor output at stoichiometry (NO xrich ) to a predetermined value. A decrease in the ratio below the predetermined value is indicative of sensor deterioration. 
     The inventors herein have recognized a disadvantage with this approach. Namely, this method would not work for a post-catalyst NOx sensor since the LNT stores NOx during lean operation, and therefore the NO xlean  signal downstream of the LNT will be attenuated. Therefore, the ratio of the prior art cannot be used as an indicator of post-catalyst sensor deterioration. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to provide a system and a method for determining degradation in an emission control system. 
     In carrying out the above object and other objects, features and advantages of the present invention, a system and a method for determining degradation in an emission control system comprising an exhaust gas aftertreatment device having a downstream sensor coupled to it, include: changing an air-fuel mixture of an exhaust gas entering the device; and determining degradation of the sensor based on a response of the sensor to said change in said air-fuel ratio. 
     For example, in accordance with one embodiment of the present invention, the inventors have recognized that once lean operation is discontinued, and the air-fuel ratio of the exhaust gas entering the LNT is switched to rich, there is a significant temporary increase in the amount of NOx in the exhaust gas exiting the LNT, which is typically reflected by a surge in the NOx sensor output if the NOx sensor is not degraded. Further, the inventors have recognized that a degraded sensor will not detect this surge in the amount of NOx exiting the LNT. The NOx surge can occur within a predetermined time period following the lean to rich transition, and may sometimes happen after the purge is completed, and the air-fuel ratio is changed back to lean. It is partially due to a significant temperature increase of the LNT resulting from the increased amount of reductant entering it during the purge. In other words, increased LNT temperature contributes to increased NOx in the exhaust gas exiting the LNT. Therefore, under the present invention, the performance of the NOx sensor can be monitored by monitoring its response to the NOx surge for a predetermined time period following the switch from lean to rich operation. 
     In accordance with another feature of the present invention, in an exemplar embodiment, a system and a method for determining degradation in an emission control system comprising an exhaust gas aftertreatment device having a downstream sensor coupled to it, include: operating the engine at an air-fuel ratio lean of stoichiometry to store an exhaust gas constituent in the device; temporarily switching to an air-fuel ratio rich of stoichiometry to release said stored exhaust gas constituent from the device; reading an output of the sensor for a predetermined period following said temporary switch to determine a maximum value and a minimum value of said reading; and comparing a ratio of said maximum value and said minimum value to a predetermined threshold. 
     Therefore, according to this embodiment, it is possible to detect deterioration in the sensor by determining a maximum and a minimum value of the sensor reading following a switch to rich mode of operation, and by comparing the ratio of the two to a predetermined threshold indicative of a borderline sensor performance. In other words, a sensor that is not deteriorated will detect a surge in the amount of NOx exiting the LNT in response to a switch to rich operation, and the ratio of the maximum value to a minimum value taken during a predetermined period following the switch will be above a threshold amount. On the other hand, a deteriorated sensor will not detect the surge, and the ratio of maximum to minimum value will be below a predetermined threshold. 
     An advantage of the above aspects of invention is optimized lean running time, increased fuel economy, and improved emission control. 
     The above advantages and other advantages, objects and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The objects and advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Description of Preferred Embodiment, with reference to the drawings, wherein: 
     FIG. 1 is a block diagram of an internal combustion engine illustrating various components related to the present invention; 
     FIG. 2 is a block diagram of the embodiment in which the invention is used to advantage; 
     FIG. 3 depicts the desired engine air-fuel ratio versus time; 
     FIG. 4 depicts outputs of a non-deteriorated NOx sensor versus time in response to a change in air-fuel ratio; and 
     FIG. 5 depicts outputs of a deteriorated NOx sensor versus time in response to a change in air-fuel ratio. 
    
    
     DETAILED DESCRIPTION 
     As will be appreciated by those of ordinary skill in the art, the present invention is independent of the particular underlying engine technology and configuration. As such, the present invention may be used in a variety of types of internal combustion engines, such as conventional engines, in addition to direct injection stratified charge (DISC) or direct injection spark ignition engines (DISI). 
     A block diagram illustrating an engine control system and method for a representative internal combustion engine according to the present invention is shown in FIG.  1 . Preferably, such an engine includes a plurality of combustion chambers, only one of which is shown, and is controlled by electronic engine controller  12 . Combustion chamber  30  of engine  10  includes combustion chamber walls  32  with piston  36  positioned therein and connected to crankshaft  40 . In this particular example, the piston  30  includes a recess or bowl (not shown) for forming stratified charges of air and fuel. In addition, combustion chamber  30  is shown communicating with intake manifold  44  and exhaust manifold  48  via respective intake valves  52   a  and  52   b  (not shown), and exhaust valves  54   a  and  54   b  (not shown). A fuel injector  66  is shown directly coupled to combustion chamber  30  for delivering liquid fuel directly therein in proportion to the pulse width of signal fpw received from controller  12  via conventional electronic driver  68 . Fuel is delivered to the fuel injector  66  by a conventional high-pressure fuel system (not shown) including a fuel tank, fuel pumps, and a fuel rail. 
     Intake manifold  44  is shown communicating with throttle body  58  via throttle plate  62 . In this particular example, the throttle plate  62  is coupled to electric motor  94  such that the position of the throttle plate  62  is controlled by controller  12  via electric motor  94 . This configuration is commonly referred to as electronic throttle control, (ETC), which is also utilized during idle speed control. In an alternative embodiment (not shown), which is well known to those skilled in the art, a bypass air passageway is arranged in parallel with throttle plate  62  to control inducted airflow during idle speed control via a throttle control valve positioned within the air passageway. 
     Exhaust gas sensor  76  is shown coupled to exhaust manifold  48  upstream of catalytic converter  70 . In this particular example, sensor  76  is a universal exhaust gas oxygen (UEGO) sensor, also known as a proportional oxygen sensor. The UEGO sensor generates a signal whose magnitude is proportional to the oxygen level (and the air-fuel ratio) in the exhaust gases. This signal is provided to controller  12 , which converts it into a relative air-fuel ratio. Advantageously, signal UEGO is used during feedback air-fuel ratio control in to maintain average air-fuel ratio at a desired air-fuel ratio as described later herein. In an alternative embodiment, sensor  76  can provide signal EGO, exhaust gas oxygen (not shown), which indicates whether exhaust air-fuel ratio is lean or rich of stoichiometry. In another alternate embodiment, the sensor  76  may comprise one of a carbon monoxide (CO) sensor, a hydrocarbon (HC) sensor, and a NOx sensor that generates a signal whose magnitude is related to the level of CO, HC, NOx, respectively, in the exhaust gases. Those skilled in the art will recognize that any of the above exhaust gas sensors may be viewed as an air-fuel ratio sensor that generates a signal whose magnitude is indicative of the air-fuel ratio measured in exhaust gases. 
     Conventional distributorless ignition system  88  provides ignition spark to combustion chamber  30  via spark plug  92  in response to spark advance signal SA from controller  12 . 
     Controller  12  causes combustion chamber  30  to operate in either a homogeneous air-fuel ratio mode or a stratified air-fuel ratio mode by controlling injection timing. In the stratified mode, controller  12  activates fuel injector  66  during the engine compression stroke so that fuel is sprayed directly into the bowl of piston  36 . Stratified air-fuel layers are thereby formed. The stratum closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. In the homogeneous mode, controller  12  activates fuel injector  66  during the intake stroke so that a substantially homogeneous air-fuel mixture is formed when ignition power is supplied to spark plug  92  by ignition system  88 . Controller  12  controls the amount of fuel delivered by fuel injector  66  so that the homogeneous air-fuel ratio mixture in chamber  30  can be selected to be substantially at (or near) stoichiometry, a value rich of stoichiometry, or a value lean of stoichiometry. Operation substantially at (or near) stoichiometry refers to conventional closed loop oscillatory control about stoichiometry. The stratified air-fuel ratio mixture will always be at a value lean of stoichiometry, the exact air-fuel ratio being a function of the amount of fuel delivered to combustion chamber  30 . 
     An additional split mode of operation, wherein additional fuel is injected during the exhaust stroke while operating in the stratified mode, is available. An additional split mode of operation wherein additional fuel is injected during the intake stroke while operating in the stratified mode is also available, where a combined homogeneous and split mode is available. 
     Lean NOx Trap  72  is shown positioned downstream of catalytic converter  70 . Both devices store exhaust gas components, such as NO X  and oxidants, when engine  10  is operating lean of stoichiometry. The stored exhaust gas components are subsequently reacted with HC and other reductant and are catalyzed during a purge cycle when controller  12  causes engine  10  to operate in either a rich mode or a near 
     Exhaust gas oxygen sensor  150  also known as a catalyst monitoring sensor (CMS) is shown coupled to exhaust manifold  48  between the catalytic converter  70  and the NOx trap  72 . In this particular example, sensor  150  provides signal HEGO to controller  12 , and essentially serves as a switch providing information as to whether the air-fuel mixture is lean or rich at the mid-bed location. 
     Controller  12  is shown in FIG. 1 as a conventional microcomputer including but not limited to: microprocessor unit  102 , input/output ports  104 , an electronic storage medium for executable programs and calibration values, shown as read-only memory chip  106  in this particular example, random access memory  108 , keep alive memory  110 , and a conventional data bus. 
     Controller  12  is shown receiving various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including: measurement of inducted mass air flow (MAF) from mass air flow sensor  100  coupled to throttle body  58 ; engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling sleeve  114 ; a profile ignition pickup signal (PIP) from Hall effect sensor  118  coupled to crankshaft  40  giving an indication of engine speed (RPM); throttle position TP from throttle position sensor  120 ; and absolute Manifold Pressure Signal MAP from sensor  122 . Engine speed signal RPM is generated by controller  12  from signal PIP in a conventional manner and manifold pressure signal MAP provides an indication of engine load. 
     Fuel system  130  is coupled to intake manifold  44  via tube  132 . Fuel vapors (not shown) generated in fuel system  130  pass through tube  132  and are controlled via purge valve  134 . Purge valve  134  receives control signal PRG from controller  12 . 
     Exhaust sensor  140  is a NOx/UEGO sensor located downstream of the LNT. It produces two output signals. Both first output signal (SIGNAL 1 ) and second output signal (SIGNAL 2 ) are received by controller  12 . Exhaust sensor  140  can be a sensor known to those skilled in the art that is capable of indicating both exhaust air-fuel ratio and nitrogen oxide concentration. 
     In a preferred embodiment, SIGNAL 1  indicates exhaust air-fuel ratio and SIGNAL 2  indicates nitrogen oxide concentration. In this embodiment, sensor  140  has a first chamber (not shown) in which exhaust gas first enters where a measurement of oxygen partial pressure is generated from a first pumping current. Also, in the first chamber, oxygen partial pressure of the exhaust gas is controlled to a predetermined level. Exhaust air-fuel ratio can then be indicated based on this first pumping current. Next, the exhaust gas enters a second chamber (not shown) where NO X  is decomposed and measured by a second pumping current using the predetermined level. Nitrogen oxide concentration can then be indicated based on this second pumping current. In an alternative embodiment, a separate NOx sensor could be used in conjunction with an air-fuel sensor, which could be a UEGO or a HEGO sensor. 
     The diagram in FIG. 2 generally represents operation of one embodiment of a system or method according to the present invention. As will be appreciated by one of ordinary skill in the art, the diagram may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various steps or functions illustrated may be performed in the sequence illustrated, I parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the objects, features and advantages of the invention, but is provided for ease of illustration and description. Although not explicitly illustrated, one of ordinary skill in the art will recognize that one or more of the illustrated steps or functions may be repeatedly performed depending on the particular strategy being used. 
     Referring now to FIG. 2, the routine begins in step  50  wherein a determination is made whether any NOx sensor electrical failure (i.e., short to power or ground, or open circuit) has been detected. If the answer to step  50  is YES, a diagnostic code is set in step  100 , and the routine proceeds to step  150  wherein engine operation is switched to stoichiometric in the absence of a properly functioning NOx sensor, and the routine exits. If the answer to step  50  is NO, the routine proceeds to step  200  wherein a lean-burn mode of operation (i.e., engine operation is at an air-fuel ratio lean of stoichiometry) commences. Next, in step  250 , a determination is made whether engine operation has been switched to rich. The switch to air fuel ratio rich of stoichiometric could be due to, for example, the LNT being saturated with NOx, or to an increased demand in engine output torque. If the answer to step  250  is YES, the routine proceeds to step  300 , the output of the NOx sensor (in this example, SIGNAL 2  of NOx/UEGO sensor  140 , as described in FIG. 1) is monitored in order to determine a maximum NOx sensor value (NOx max ) following the switch. The routine then proceeds to step  350  wherein a determination is made whether the rich operation has been discontinued, due to, for example, completion of the purge of the LNT, or decreased engine output torque demand. If the answer to step  350  is NO, i.e., rich operation has not been discontinued, the routine returns to step  300 , wherein monitoring of the NOx sensor output for a maximum value continues. If the answer to step  350  is YES, i.e., rich operation is discontinued, the routine proceeds to step  400  wherein a calibratable timer is set, and then to step  450  wherein the NOx sensor output continues to be monitored for a maximum value and also monitoring for a minimum value of NOx sensor output, (NOx min ) commences. Next, in step  500 , a determination is made whether the timer set in step  400  has exceeded a predetermined value. If the answer to step  500  is NO, the routine returns to step  450 . If the answer to step  500  is YES, the routine proceeds to step  550  wherein a decision is made whether the ratio of NOx max /NOx min  is greater than a predetermined threshold. If the answer to step  550  is YES, sensor performance is not degraded, and the routine returns to step  100  wherein monitoring continues. If the answer to step  550  is NO, sensor performance is degraded, routine returns to step  150 . 
     Therefore, according to the present invention, it is possible to diagnose degradation in an exhaust sensor coupled downstream of an emission control device by varying the air-fuel ratio of the exhaust gas mixture entering the device and comparing the response of the sensor to the change in the air-fuel ratio to a predicted response. In one of the embodiments, the response of the sensor to a switch from lean to rich air-fuel ratio is monitored by determining a ratio of a maximum and minimum sensor reading during a predetermined period following the switch and comparing the ratio to a predetermined threshold. If the sensor performance is judged degraded, the engine control strategy could be adjusted, for example, by changing the air-fuel ratio to stoic. 
     Proceeding now to FIG. 3, an exemplary plot of desired engine air-fuel ratio is depicted. As can be seen in the plot, at time t 1  the air fuel ratio is changed from lean to rich (due to, for example, driver demand for extra torque, or to the LNT being saturated with NOx). At time t 2  , rich operation is discontinued (i.e., NOx purge is completed) and lean operation resumes. 
     Referring now to FIG. 4, an exemplary plot of a properly functioning NOx sensor response to changes in the desired air-fuel ratio is depicted. The NOx max /NOx min  ratio is above a threshold value which in this example is around 500 ppm. 
     Referring now to FIG. 5, an exemplary plot of a degraded NOx sensor response to changes in the desired air-fuel ratio is depicted. It can be seen that the NOx  max /NOx min  ratio is below an exemplary 500 ppm threshold value. 
     This concludes the description of the invention. The reading of it by those skilled in the art would bring to mind many alterations and modifications without departing from the spirit and the scope of the invention. Accordingly, it is intended that the scope of the invention is defined by the following claims: