Patent Publication Number: US-7900615-B2

Title: Air-fuel imbalance detection based on zero-phase filtering

Description:
FIELD 
     The present invention relates to engine control, and more particularly to engine emission control using air-fuel imbalance detection. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Internal combustion engines compress and ignite a mixture of air and fuel in a cylinder to produce power. An imbalance in the air-fuel mixture may produce excessive emissions in exhaust gases exiting the cylinders. An oxygen concentration sensor may measure oxygen concentration levels in the exhaust gas. By measuring the oxygen concentration in the exhaust gas, the air-fuel mixture may be adjusted to improve combustion efficiency and reduce excessive emissions. 
     SUMMARY 
     Accordingly, the present disclosure provides a control system comprising an oxygen sensor that generates an oxygen signal based on an oxygen concentration level in an exhaust gas of an engine, a filtering module that determines a filtered signal based on the oxygen signal, and an air-fuel imbalance detection module that detects an air-fuel imbalance in the engine based on the oxygen signal and the filtered signal. In addition, the present disclosure provides a method comprising generating an oxygen signal based on an oxygen concentration level in an exhaust gas of an engine, determining a filtered signal based on the oxygen signal, and detecting an air-fuel imbalance in the engine based on the oxygen signal and the filtered signal. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram of a vehicle including an air-fuel imbalance system according to the present disclosure; 
         FIG. 2  is a functional block diagram of a control module according to the present disclosure; 
         FIG. 3  is a flowchart illustrating exemplary steps of an air-fuel imbalance detection method according to the present disclosure; 
         FIG. 4  illustrates exemplary signals representing oxygen content in an exhaust gas of an engine having no air-fuel imbalance; 
         FIG. 5  illustrates exemplary signals representing oxygen content in an exhaust gas of an engine having an air-fuel imbalance; and 
         FIG. 6  illustrates exemplary signals based on oxygen sensor signals indicating an air-fuel imbalance and no air-fuel imbalance. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the disclosure, 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 phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical or. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure. 
     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 , a vehicle  10  includes an engine  12 , an exhaust system  14  and a control module  16 . Air is drawn into the engine through an intake manifold  18 . The air is combusted with fuel inside cylinders (not shown) of the engine  12 . Exhaust produced by the combustion process exits the engine  12  through the exhaust system  14 . The exhaust system  14  includes a catalytic converter  22 , a pre-catalyst or inlet oxygen (O 2 ) sensor  24  and a post-catalyst or outlet oxygen (O 2 ) sensor  26 . The exhaust gas is treated in the catalytic converter  22  and is released to atmosphere. 
     The inlet and outlet O 2  sensors  24 ,  26  generate signals based on the O 2  content of the exhaust gas. The signals are communicated to the control module  16 . The control module  16  determines the A/F ratio based on the signals. The control module  16  communicates with a fuel system  28 , which regulates fuel flow to the engine  12 . In this manner, the control module  16  adjusts and regulates the A/F ratio to the engine  12 . 
     The inlet and outlet O 2  sensors  24 ,  26  are typically narrow range switching sensors. It is appreciated, however, that the inlet and outlet O 2  sensors  24 ,  26  are not limited to narrow range type switching sensors. Voltage output signals that are generated by the O 2  sensors  24 ,  26  are based on the O 2  content of the exhaust passing the O 2  sensors relative to stoichiometry. The signals transition between lean and rich in an A/F ratio range that brackets the stoichiometric A/F ratio. The O 2  sensor signal that is generated by the inlet O 2  sensor  24  switches back and forth between rich and lean values. 
     The control module  16  regulates the fuel flow based on the O 2  sensor signals. For example, if the inlet O 2  sensor signal indicates a lean condition, the control module  16  increases fuel flow to the engine  12 . Conversely, if the inlet O 2  sensor signal indicates a rich condition, the control module  16  decreases fuel flow to the engine  12 . The amount of fuel is determined based on fuel offset gains, which are determined based on the sensor signals. 
     An air-fuel imbalance in the engine  12  causes fast switching of the O 2  sensor  24 , yielding a high frequency O 2  sensor signal. The amount of air flowing through the intake manifold  18  and the rotational speed of the engine  12  may cause undesired exhaust gas separation. Depending on sensitivity level of the O 2  sensor  24 , exhaust gas separation may cause O 2  sensor signal noise and false diagnosis of an air-fuel imbalance. The air-fuel imbalance detection system and method of the present disclosure has a sufficient signal-to-noise (S/N) ratio to prevent false diagnosis of an air-fuel imbalance. 
     The air-fuel imbalance detection system and method of the present disclosure detects an air-fuel imbalance in the engine  12  based on an O 2  sensor signal. More specifically, the air-fuel imbalance detection system and method filters the O 2  sensor signal and detects an air-fuel imbalance based on the unfiltered O 2  sensor signal and the filtered O 2  sensor signal. The air-fuel imbalance detection system and method employs a filter that removes any high-frequency imbalance from the unfiltered O 2  sensor signal such that the unfiltered and filtered O 2  sensor signals may be used to identify an air-fuel imbalance. A sufficient S/N ratio is achieved through a filter that removes any high-frequency imbalance but does not remove noise due to sensitivity of the O 2  sensor  24 . 
     The control module  16  detects an air-fuel imbalance according to the principles of an air-fuel imbalance detection system and method of the present disclosure. When the engine  12  is running, the control module  16  filters the O 2  sensor signal using a zero-phase, low-pass digital filter to obtain the filtered O 2  sensor signal. The control module  16  calculates a difference between the O 2  sensor signal and the filtered O 2  sensor signal and calculates a variance based on the difference to yield an index that indicates an air-fuel imbalance level. When the index exceeds a predetermined threshold, the control module  16  detects an air-fuel imbalance. 
     Referring now to  FIG. 2 , the control module  16  includes a filtering module  200  and an air-fuel imbalance detection module  202 . The filtering module  200  receives the O 2  sensor signal from the pre-catalyst O 2  sensor  24 . The filtering module  200  filters the O 2  sensor signal using a low-pass filter to yield a filtered O 2  sensor signal. The low-pass filter removes high frequency content indicative of an air-fuel imbalance from the O 2  sensor signal. The low-pass filter is also a zero-phase filter, or a filter having precisely zero-phase distortion. 
     The air-fuel imbalance detection module  202  receives the unfiltered O 2  sensor signal from the pre-catalyst O 2  sensor  24  and the filtered O 2  sensor signal from the filtering module  200 . The air-fuel imbalance detection module  202  calculates a difference between the unfiltered and filtered O 2  sensor signals and determines a variance of the difference. More specifically, the air-fuel imbalance detection module  202  sets the variance equal to the square of the difference between the unfiltered and filtered O 2  sensor signals. 
     The air-fuel imbalance detection module  202  determines an index of an air-fuel imbalance level based on the variance. More specifically, the air-fuel imbalance detection module  202  may set the index equal to the variance. Alternatively, the air-fuel imbalance detection module  202  may filter the variance and set the index equal to the filtered variance to avoid false detection of an air-fuel imbalance due to variations in an unfiltered index. The air-fuel imbalance detection module  202  determines whether the index exceeds a predetermined threshold. When the index exceeds the predetermined threshold, the air-fuel imbalance detection module  202  detects an air-fuel imbalance and generates a service indicator signal. 
     Referring now to  FIG. 3 , exemplary steps of an air-fuel imbalance detection method according to the present disclosure will be described. In step  300 , control generates an O 2  sensor signal based on an O 2  concentration level in an exhaust gas of an engine. In step  302 , control filters the O 2  sensor signal to obtain a filtered O 2  sensor signal. In steps  304  through  310 , control detects an air-fuel imbalance based on the unfiltered and filtered O 2  sensor signals. 
     In step  304 , control determines a difference between the unfiltered and filtered O 2  sensor signals. In step  306 , control determines an index of an air-fuel imbalance level based on a variance or square of the difference. More specifically, control may set the index equal to the variance. Alternatively, control may filter the variance and set the index equal to the filtered variance to avoid false detection of an air-fuel imbalance due to variations in an unfiltered index. 
     In step  308 , control determines whether the index of the air-fuel imbalance level exceeds a predetermined air-fuel imbalance level threshold. When the index exceeds the threshold, control detects an air-fuel imbalance in step  310 . For robustness (i.e., avoidance of false air-fuel imbalance detection), control may detect the air-fuel imbalance when the index exceeds the threshold for a predetermined time period. Control may set a service indicator, such as a diagnostic trouble code (DTC), when an air-fuel imbalance is detected. Since O 2  sensors typically measure O 2  content of exhaust gas exiting a single bank of cylinders, control may set independent service indicators for each bank. 
     Referring now to  FIG. 4 , exemplary raw (i.e., unfiltered) and filtered O 2  sensor signals indicative of an engine having no air-fuel imbalance are illustrated. The y-axis represents the O 2  sensor output, and the x-axis represents the time period that the O 2  sensor signal was monitored to detect an air-fuel imbalance. Variation between the raw and filtered O 2  sensor signals is minimal. In addition, no phase shift exists between the filtered and unfiltered O 2  sensor signals as a zero-phase filter was used to obtain the filtered O 2  sensor signal. 
     Referring now to  FIG. 5 , exemplary raw and filtered O 2  sensor signals indicative of an engine having an air-fuel imbalance are illustrated. The y-axis represents the O 2  sensor output, and the x-axis represents the time period that the O 2  sensor signal was monitored to detect an air-fuel imbalance. In the graph on the left, a moderate amount of variation exists between the raw and filtered O 2  sensor signals due to a moderate amount of air-fuel imbalance. In the graph on the right, a significant amount of variation exists between the raw and filtered O 2  sensor signals due to a significant amount of air-fuel imbalance. 
     Referring now to  FIG. 6 , exemplary post-processed signals indicative of an engine having an air-fuel imbalance and an engine having no air-fuel imbalance are illustrated. In the graph on the left, the y-axis represents a residual (i.e., difference) between the unfiltered and filtered O 2  sensor signals and the x-axis represents a time period during which the O 2  sensor signal was monitored to detect an air-fuel imbalance. The graph on the left compares a passing residual (i.e., does not indicate an air-fuel imbalance) and a failing residual (i.e., indicates an air-fuel imbalance). While the passing residual is near 0 mV for a majority of the monitored time period, the failing residual exhibits several spikes with magnitudes exceeding 300 mV. 
     In the graph on the right, the y-axis represents a variance of the residual between the unfiltered and filtered O 2  sensor signals and the x-axis represents the number of samples from the O 2  sensor signal monitored to detect an air-fuel imbalance. The graph on the right compares a passing variance (i.e., does not indicate an air-fuel imbalance) and a failing variance (i.e., indicates an air-fuel imbalance). The passing variance remains relatively constant compared to the failing variance, and the magnitude of the passing variance is significantly lower than the magnitude of the failing variance. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure 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.