Abstract:
A non-intrusive method and arrangement for detecting the aging of an oxygen sensor, without increasing tailpipe emissions, is provided. The method detects an aging oxygen sensor, located between a motor vehicle engine and a catalytic converter, by sampling a series of oxygen level signals taken over a calibratable time block only when at least one engine operating condition. satisfies a predetermined criterion whereunder the method will not intrude upon the engine controller&#39;s ability to minimize undesirable exhaust emissions. After a series of signal processing, the samplings are then compared to calibratable thresholds in order to determine the aging degree of the oxygen sensor.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to monitoring air-fuel compositions using an oxygen sensor, and in particular, to a method for detecting an aging oxygen sensor.  
         BACKGROUND OF THE INVENTION  
         [0002]    The United States government stringently regulates motor vehicle emission levels for pollutants such as carbon monoxide (CO), hydrocarbons (HC), and oxides of nitrogen (NO x ). Engine performance and pollutant emissions depend upon the air-fuel mixture supplied to an engine.  
           [0003]    A fuel metering system, that monitors oxygen levels in the exhaust gases, controls the quantity of fuel contained in the air-fuel mixture. An oxygen sensor, located between the motor vehicle engine and the catalytic converter in the engine exhaust system, provides precision feedback to the metering system enabling it to make immediate adjustments to the air-fuel mixture. Accurate feedback from the oxygen sensor to the fuel metering system is essential for proper regulation of the level of pollutants in motor vehicle exhaust gases. Such accuracy, in turn, requires a properly functioning oxygen sensor.  
           [0004]    Due to the proximity of the oxygen sensor to the vehicle engine, exhaust gases contacting the sensor are very hot and chemically active—conditions which cause aging of the sensor. Hence, vehicles have used a variety of methods of attempting to ascertain whether a sensor has aged to the point of requiring replacement.  
           [0005]    Known diagnostic routines for monitoring performance of exhaust stream oxygen sensors are “intrusive”—i.e., such routines may interfere with, or intrude upon, an engine control module&#39;s normal fuel metering functions for minimizing undesirable exhaust emissions. Such conventional diagnostics likewise intrude upon a control module&#39;s capability to optimize a variety of drivability characteristics of the vehicle.  
           [0006]    Hence, there is seen to be a need for a non-intrusive diagnostic method for judging whether an exhaust gas oxygen sensor requires replacement.  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly, a method for detecting proper functioning of an oxygen sensor mounted in an engine exhaust stream of a vehicle and in communication with an engine controller is initiated only when at least one engine operating condition satisfies a predetermined criterion whereunder the method will not intrude upon the engine controller&#39;s ability to minimize undesirable exhaust emissions. Once the criterion is satisfied, at least one mathematical characteristic is determined from a sequence of readings of an output of the sensor over a predetermined time interval. The at least one characteristic is compared to a corresponding test standard, and proper sensor functioning is determined whenever the at least one characteristic compares favorably to its corresponding standard.  
           [0008]    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  
       [0009]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0010]    [0010]FIG. 1 is a block diagram showing the components of an oxygen sensor monitoring arrangement in accordance with the present invention;  
         [0011]    [0011]FIGS. 2A and 2B are flow charts depicting a method of detecting an aging oxygen sensor in accordance with the present invention; and  
         [0012]    [0012]FIG. 3 is a diagram illustrating a sampling block of the output signal from an oxygen sensor in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0014]    [0014]FIG. 1 illustrates an oxygen sensor monitoring arrangement  10  having a motor vehicle engine  14 , a catalytic converter  18 , an oxygen sensor  16 , and an engine control module  12 . The oxygen sensor  16  monitors the level of oxygen in exhaust gases between the motor vehicle engine  14  and catalytic converter  18 . An acceptability of the aging of sensor  16  is monitored via a non-intrusive diagnostic routine resident in microprocessor-based engine control module  12 .  
         [0015]    A method for detecting the aging of an oxygen sensor using oxygen sensor monitoring arrangement  10  is shown in FIGS. 2A and 2B. As seen in FIG. 2A, upon starting the routine at step  200 , various accumulator registers discussed below are cleared at step  202 . To ensure a non-intrusive nature of the routine, an enablement and stabilization decision test  204  is performed. Generally, an enablement routine checks a range of vehicle operating conditions including, but not limited to, engine rotational speed, engine coolant temperature, and maintenance of an unbiased (i.e. neither rich nor lean) air/fuel ratio. If enablement conditions are not met, the routine will not continue until the selected engine operating conditions are acceptable. When conditions are determined to be acceptable, test block accumulations are cleared at step  206  and a block testing window begins at step  208 .  
         [0016]    Referring to FIG. 3, the routine of FIGS. 2A and 2B monitors the output of the oxygen sensor  16  over a plurality of test time blocks, one block being shown as  24 . A block is defined as a calibratable or preselected number of sensor output samples in the form of electrical signals, obtained from oxygen sensor  16 . At step  210  of FIG. 2A, oxygen sensor  16  provides sequential samplings  42 ,  44 , in the form of electrical signals at sampling times T 1  to T N  of FIG. 3 that reflect the level of oxygen in the exhaust gases for each sampling  42 , 44 .  
         [0017]    At step  212 , the oxygen level samplings are summed and a maximum and minimum sampling value is determined for the current block. The oxygen voltage levels at consecutive pairs of samplings between T 1  and T N  are then used at step  214  to calculate the absolute value of the change in sensor signal level from one sample to the next. This difference is referred to herein as the delta voltage. For example, delta voltage  46  of FIG. 3 between time T 1  and T 2  is 0.50, and is calculated by taking the absolute value of the difference between the oxygen level sampling value at point  44  (0.75) and the oxygen sampling value at point  42  (0.25).  
         [0018]    The oxygen level samplings, delta voltages, maximum sampling, and minimum sampling, constitute the testing parameters of the diagnostic routine.  
         [0019]    Once the delta voltages are calculated at step  214 , a sequence of signal conditioning processes and quality checks is performed. At step  216  the engine exhaust flow rates for samplings within block  24  of FIG. 3 are filtered using a low pass filter to obtain an average flow rate for block  24 . In each test block, delta signals are influenced by the exhaust flow rate. The higher the flow rate, the greater the change, or delta signal, between sensor readings. Hence, in order to properly compensate delta values, to be explained below, each delta signal obtained during a given block is added to one of a plurality of delta accumulators, each accumulator being associated with a predefined range of average exhaust flow rates.  
         [0020]    At step  218  an engine condition check is performed to verify that engine  14  has not experienced any abrupt changes in manifold pressure that could compromise the oxygen sensor output data for the current block. If undesirable pressure changes have occurred, the routine is aborted and returns to the enablement and stabilization step  204 . Otherwise, at step  220  a quality check is applied to the delta signals to minimize noise and quantization errors. If the quality check fails, the delta signals for the current block are ignored by skipping storage step  222 . A quality check in its simplest form would look for excessive delta signals indicative of noise, by looking for delta signals exceeding a preselected limit value and discarding same.  
         [0021]    At step  222 , the delta signal summations for the current block are assigned to one of a plurality of block accumulators based on the previously determined average exhaust flow rate for the current block.  
         [0022]    At step  224 , a check for the expiration of the block testing timer is performed. If the block timer is not full, the system returns to step  208  to continue testing for the current block.  
         [0023]    If the block timer at step  224  has expired, the current block is finished, and at step  226  the data for the samplings, the maximum samplings, and the minimum samplings are stored in total accumulators. At step  228 , the accumulated delta signals for the current block are assigned to one of a plurality of total delta signal accumulators based on the average exhaust flow rate for the just-completed block.  
         [0024]    At step  230 , if the total for the block test counter is not reached, another block test begins at step  206 . If, however, the block counter maximum is reached, the total sampling time has expired, and flow rate compensation is performed on all delta signals so that all the data used is normalized to a nominal flow rate. Normalization is effected by increasing all delta signals calculated at flow rates below the nominal rate and by decreasing all delta signals calculated at flow rates greater than the nominal rate.  
         [0025]    Proceeding to FIG. 2B, at step  234 , the block samplings for the total number of blocks are summed individually and used to calculate an average block sampling signal value. At step  236 , average values are likewise calculated for the normalized delta signal accumulations and for the maximum and minimum sample values for all blocks.  
         [0026]    The parameter averages are then compared to thresholds, or test standards, to determine whether the oxygen sensor has aged to the extent of needing replacement.  
         [0027]    At step  238 , the average sampling is compared to a calibratable sampling threshold range. If the average sampling is not within the threshold range, the sensor is considered, at step  248 , to have aged to an unacceptable degree. If the average sampling, however, falls within the threshold range, the sensor, at least from an average sampling standpoint, is considered acceptable.  
         [0028]    A similar process is repeated for the remaining parameters. At step  240 , the average delta signal is compared to a calibratable delta voltage threshold. If the average delta voltage is less than the delta voltage threshold, the sensor is considered, at step  248 , to have aged to an unacceptable degree. If the average delta voltage is greater than the voltage threshold, the sensor, at least from an average delta voltage standpoint, is considered acceptable.  
         [0029]    At step  242 , the average maximum sampling is compared to a calibratable maximum sampling threshold. If the average maximum sampling is less than the maximum sampling threshold, the sensor is considered at step  248  to have aged to an unacceptable degree. If the average maximum sampling, however, is greater than the maximum sampling threshold, the sensor, at least from a maximum sampling standpoint is considered acceptable.  
         [0030]    Finally, at step  244 , the average minimum sampling is compared to a minimum sampling threshold. If the average minimum sampling is greater than the minimum sampling threshold, the sensor is considered, at step  248 , to have aged to an unacceptable degree. However, if the average minimum sampling is less than the minimum sampling threshold, the sensor is considered, at step  246 , to be within an acceptable range.  
         [0031]    It is to be understood that, preferably, all four tests  238 ,  240 ,  242  and  244  pass, i.e. the associated average compares favorably to the standard, in order for a sensor to be deemed acceptably functioning. However, under appropriate conditions, one or more of the above four tests may be eliminated in conducting sensor diagnosis.  
         [0032]    The description of the preferred embodiment is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention, as determined from proper interpretation of the appended claims.