Abstract:
A catalytic monitoring method for an engine having two engine banks of which each coupled to one of two catalytic converters using first and second exhaust gas oxygen sensors respectively, upstream and downstream of one catalytic converter. Third and fourth exhaust gas oxygen sensors are respectively coupled upstream and downstream of the other catalytic converter. Switch ratios are determined for each of the engine banks based on the switching ratios of each upstream and downstream pair of exhaust gas oxygen sensors. A combination of the switch ratios is used to determine overall catalytic converter system performance.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/175,111, filed on Oct. 19, 1998 (now U.S. Pat. No. 6,151,889), entitled “Catalytic Monitoring Method”. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to catalytic converter monitoring systems of internal combustion engines responsive to exhaust gas oxygen sensors before and after the catalytic converters. 
     BACKGROUND OF THE INVENTION 
     To meet current emission regulations, automotive vehicles must have on-board diagnostic systems to detect the malfunction of any component of the emission system, including a catalytic converter. A vehicle containing more than one catalytic converter may monitor each catalytic converter to guarantee detection of degraded system performance. An example would be that of a vehicle containing a V-type engine where catalytic converters coupled near or close to the engine are used. In this system, each catalytic converter may be monitored to determine emission compliance. 
     Catalytic converter monitoring systems are known in which an upstream and a downstream exhaust gas oxygen sensor are compared to give an indication of catalytic degradation. If sufficient degradation occurs, these systems will diagnose the deteriorated performance and illuminate a malfunction indicator. As a result, a new catalytic converter will be required. In the example of a V-type engine using two close coupled catalytic converters, each being monitored, when the measured performance of either of the two catalytic converters falls below a predetermined threshold, a malfunction is indicated. An example of such an approach is disclosed in U.S. Pat. No. 5,357,751. 
     The inventors herein have recognized numerous problems with the above approach. For example, in systems containing a dual bank engine connected to two monitored catalytic converters, the catalytic converters may not age at the same rate. Thus, one catalytic converter may have degraded past a threshold indicating possible reduced performance while the other is operating with much higher performance. While the total emissions of the vehicle are still within allowable amounts, a malfunction is indicated because one of the catalytic converters has degraded much faster than the other. 
     SUMMARY OF THE INVENTION 
     An object of the invention claimed herein is to provide a catalytic converter monitoring method capable of accounting for variations in catalytic converter aging between two converters, each coupled to separate engine banks. 
     The above object is achieved, and problems of prior approaches overcome, by the method shown in claim 1. In one particular aspect of the invention, the method comprises measuring a first number of transitions from a first state to a second state of a first exhaust gas oxygen sensor upstream of the first catalytic converter, measuring a second number of transitions from said first state to said second state of a second exhaust gas oxygen sensor downstream of the first catalytic converter, measuring a third number of transitions from said first state to said second state of a third exhaust gas oxygen sensor upstream of the second catalytic converter, measuring a fourth number of transitions from said first state to said second state of a fourth exhaust gas oxygen sensor downstream of the second catalytic converter, and determining a degradation of the first and second catalytic converters derived from a combination of a first ratio between said first and second number of transitions and a second ratio between said third and number of transitions. 
     By using a combination of the first ratio and second ratio, a total system performance can be inferred, leading to more accurate catalytic converter monitoring. For example, when the first upstream and first downstream exhaust gas oxygen sensors are coupled to one bank of an engine and the second upstream and second downstream exhaust gas oxygen sensors are coupled to another bank, the combination of the first and second ratios can detect when the catalytic converters are aging unequally. This information can be used to provide a malfunction indication when the total system degradation has reached a predetermined level. 
     An advantage of the present invention is the ability to more accurately determine the total catalytic converter system performance for systems having multiple converters. 
     Another advantage of the present invention is the reduction in false malfunction indications. 
     Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification. 
    
    
     BRIEF DESCRIPTION OF THE 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 embodiment wherein the invention is used to advantage; 
     FIGS. 2-8 are high level flow charts of various operations performed by a portion of the embodiment shown in FIG. 1; and 
     FIG. 9 is a graphical representation of an example of an aspect of the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Controller  10  is shown in the block diagram of FIG. 1 as a conventional microcomputer including: microprocessor unit  12 ; input ports (not shown), output ports  16 , read-only memory  18 , for storing the control program; random access memory  20  for temporary data storage which may also be used for counters or timers; keep-alive memory  22 , for storing learned values; and a conventional data bus. 
     Controller  10  is shown receiving various signals from sensors coupled to engine  28  including; measurement of inducted mass airflow (MAF) from mass airflow sensor  32 ; engine coolant temperature (T) from temperature sensor  40 ; and indication of engine speed (rpm) from tachometer  42 . In this example, engine  28  is a V-type engine having first and second banks (not shown) coupled to respective first and second exhaust manifolds ( 57 , 56 ). 
     Output signal FEGO 1  from conventional exhaust gas oxygen sensor  45 , positioned upstream of first catalytic converter  51 , is compared to a reference value associated with stoichiometry in comparator  48  for providing output signal FEGO 1 S. Signal FEGO 1 S is a two-state signal which is a predetermined high voltage when exhaust gases are rich of stoichiometry and a predetermined low voltage when exhaust gases are lean of stoichiometry. Both signal FEGO 1  and signal FEGO 1 S are coupled to controller  10 . 
     Output signal FEGO 2  from conventional exhaust gas oxygen sensor  44 , positioned upstream of second catalytic converter  50 , is compared to a reference value associated with stoichiometry in comparator  46  for providing output signal FEGO 2 S. Signal FEGO 2 S is a two-state signal which is a predetermined high voltage when exhaust gases are rich of stoichiometry and a predetermined low voltage when exhaust gases are lean of stoichiometry. Both signal FEGO 2  and signal FEGO 2 S are coupled to controller  10 . 
     Another conventional exhaust gas oxygen sensor ( 53 ) is shown coupled to exhaust manifold  57  downstream of catalytic converter  51  and provides signal REGO 1  to controller  10  which is related to oxygen content in the exhaust gases. Output signal REGO 1  is also compared to a reference value associated with stoichiometry in comparator  55  for providing two-state output signal REGO 1 S to controller  10 . Signal REGO 1 S is preselected high voltage when exhaust gases downstream of catalytic converter  51  are rich of stoichiometry and a low preselected voltage when such exhaust gases are lean of stoichiometry. 
     Yet another conventional exhaust gas oxygen sensor ( 52 ) is shown coupled to exhaust manifold  56  downstream of catalytic converter  50  and provides signal REGO 2  to controller  10  which is related to oxygen content in the exhaust gases. Output signal REGO 2  is also compared to a reference value associated with stoichiometry in comparator  54  for providing two-state output signal REGO 2 S to controller  10 . Signal REGO 2 S is preselected high voltage when exhaust gases downstream of catalytic converter  50  are rich of stoichiometry and a low preselected voltage when such exhaust gases are lean of stoichiometry. 
     Referring now to FIG. 1, intake manifold  58  of engine  28  is shown coupled to throttle body  60  having primary throttle plate  62  positioned therein. Throttle body  60  is also shown having fuel injector  76  coupled thereto for delivering liquid fuel in proportion to the pulse width of signal fpw from controller  10 . Fuel is delivered to fuel injector  76  by a conventional fuel system including fuel tank  80 , fuel pump  82 , and fuel rail  84 . 
     Referring now to FIG. 2, a flowchart of a routine performed by controller  10  to generate fuel trim signal FT 1  for the first bank of engine  28  is now described. A determination is first made whether closed-loop air/fuel control is to be commenced (step  104 ) by monitoring engine operation conditions such as temperature. When closed-loop control commences, signal REGO 1 S is read from comparator  55  (step  108 ) and subsequently processed in a proportional plus integral controller as described below. 
     Referring first to step  126 , signal REGO 1 S is multiplied by gain constant GI and the resulting product added to products previously accumulated (GI * REGO 1 S i-1 ) in step  128 . Stated another way, signal REGO 1 S is integrated each sample period (i) in steps determined by gain constant GI. During step  132 , signal REGO 1 S is also multiplied by proportional gain GP. The integral value from step  128  is added to the proportional value from step  132  during addition step  134  to generate fuel trim signal FT 1 . 
     Referring now to FIG. 3, a flowchart of a routine performed by controller  10  to generate fuel trim signal FT 2  for the second bank of engine  28  is now described. A determination is first made whether closed-loop air/fuel control is to be commenced (step  204 ) by monitoring engine operation conditions such as temperature. When closed-loop control commences, signal REGO 2 S is read from comparator  54  (step  208 ) and subsequently processed in a proportional plus integral controller as described below. 
     Referring first to step  226 , signal REGO 2 S is multiplied by gain constant GI and the resulting product added to products previously accumulated (GI * REGO 2 S i-1 ) in step  228 . Stated another way, signal REGO 2 S is integrated each sample period (i) in steps determined by gain constant GI. During step  232 , signal REGO 2 S is also multiplied by proportional gain GP. The integral value from step  228  is added to the proportional value from step  232  during addition step  234  to generate fuel trim signal FT 2 . 
     The routine executed by controller  10  to generate the desired quantity of liquid fuel delivered to the first bank of engine  28  and trimming this desired fuel quantity by a feedback variable related both to sensor  45  and fuel trim signal FT 1  is now described with reference to FIG.  4 . During step  258 , an open-loop fuel quantity is first determined by dividing measurement of inducted mass airflow (MAF) by desired air/fuel ratio AFd which is typically the stoichiometric value for gasoline combustion. This open-loop fuel charge is then adjusted, in this example divided, by feedback variable FV 1 . 
     After determination that closed-loop control is desired (step  260 ) by monitoring engine operating conditions such as temperature (T), signal FEGO 1 S is read during step  262 . During step  266 , fuel trim signal FT 1  is transferred from the routine previously described with reference to FIG.  2  and added to signal FEGO 1 S to generate trim signal TS 1 . 
     During steps  270 - 278 , a conventional proportional plus integral feedback routine is executed with trimmed signal TS 1  as the input. Trim signal TS 1  is first multiplied by integral gain value KI (step  270 ), and the resulting product added to the previously accumulated products (step  272 ). That is, trim signal TS 1  is integrated in steps determined by gain constant KI each sample period (i) during step  272 . A product of proportional gain KP times trimmed signal TS 1  (step  276 ) is then added to the integration of KI * TS 1  during step  278  to generate feedback variable FV 1 . 
     The routine executed by controller  10  to generate the desired quantity of liquid fuel delivered to the second bank of engine  28  and trimming this desired fuel quantity by a feedback variable related both to sensor  44  and fuel trim signal FT 2  is now described with reference to FIG.  5 . During step  358 , an open-loop fuel quantity is first determined by dividing measurement of inducted mass airflow (MAF) by desired air/fuel ratio AFd which is typically the stoichiometric value for gasoline combustion. This open-loop fuel charge is then adjusted, in this example divided, by feedback variable FV 2 . 
     After determination that closed-loop control is desired (step  360 ) by monitoring engine operating conditions such as temperature (T), signal FEGO 2 S is read during step  362 . During step  366 , fuel trim signal FT 2  is transferred from the routine previously described with reference to FIG.  3  and added to signal FEGO 2 S to generate trim signal TS 2 . 
     During steps  370 - 378 , a conventional proportional plus integral feedback routine is executed with trimmed signal TS 2  as the input. Trim signal TS 2  is first multiplied by integral gain value KI (step  370 ), and the resulting product added to the previously accumulated products (step  372 ). That is, trim signal TS 2  is integrated in steps determined by gain constant KI each sample period (i) during step  372 . A product of proportional gain KP times trimmed signal TS 2  (step  376 ) is then added to the integration of KI * TS 2  during step  378  to generate feedback variable FV 2 . 
     An example of testing converter efficiency of the first engine bank is now described with particular reference to the flowchart shown in FIG.  6 . During step  498 , initial engine conditions are checked before entering the test cycle described below. More specifically, engine temperature (T) should be within a predetermine range, a predetermined time should have elapsed since the engine was started, and the closed-loop air/fuel control should have been operable for preselected time. 
     During steps  500 ,  504 , and  506 , the inducted airflow range in which engine  28  operating is determined. These ranges are described as range (i), range (j) . . . , range (n) for this example wherein “n” inducted airflow ranges are used to advantage. 
     Transitions between states of signal FEGO 1 S are counted to generate count signal CF 1   i , assuming that engine operation is within airflow range (i). This count is compared to maximum count CF 1   imax  during step  512 . While engine operation remains within airflow range (i), a test period of predetermined duration is generated by incrementing count CF 1   i  each transition of signal FEGO 1 S until count CF 1   i  is equal to maximum count CF 1   imax  (step  516 ). During this test period (i), count CR 1   i  is incremented each transition of signal REGO 1 S (step  518 ). Stated another way, count CR 1   i  is incremented each transition of signal REGO 1 S until count CR 1   i =CR 1   imax . 
     When engine operation is within airflow range (j) as shown in step  504 , predetermined period (j), count CF 1   j , and count CR 1   j  are determined in steps  522 ,  526 , and  528  in a manner similar to that described above for airflow range (i) with respect to steps  512 ,  516 , and  518 . Each transition in signal FEGO 1 S, count CF 1   j  is incremented until it reaches maximum count CF 1   jmax  (step  522 ). Predetermined test period (j) is thereby defined. During test period (j), count CR 1   j  is incremented each transition of signal REGO 1 S (step  528 ). 
     The above described operation occurs for each airflow range. For example, when engine  28  is operating within airflow range (n) as shown in step  506 , test period (n), count CF 1   n , and count CR 1   n  are generated as shown in steps  532 ,  536 , and  538 . 
     During step  550 , a determination is made as to whether engine  28  has operated in all airflow ranges (i . . . n) for the respective test periods (i . . . n). Stated another way, step  550  determines when each count of transitions in signal FEGO 1 S (CF 1   i , CF 1   j , . . . CF 1   n ) have reached their respective maximum values (CF 1   imax , CF 1   jmax , . . . CF 1   nmax ). 
     Each count (CF 1   i  . . . CF 1   n ) of transitions in signal FEGO 1 S for respective test periods (i . . . n) are summed in step  554  to generate total count CF 1   t . For reasons described above, the same total count CF 1   t  may be obtained by summing each maximum count (CF 1   imax  . . . CF 1   nmax ) for respective test periods (i . . . n). 
     Total count CR 1   t  is generated in step  556  by summing each count (CR 1   i  . . . CR 1   n ) for respective test periods (i . . . n). A ratio of total count CR 1   t  to total count CF 1   t  is then calculated during step  560  and all counts subsequently reset in step  562 . The total efficiency routine is called in step  564 . 
     An example of testing converter efficiency of the second engine bank is now described with particular reference to the flowchart shown in FIG.  7 . During step  598 , initial engine conditions are checked before entering the test cycle described below. More specifically, engine temperature (T) should be within a predetermine range, a predetermined time should have elapsed since the engine was started, and the closed-loop air/fuel control should have been operable for preselected time. 
     During steps  600 ,  604 , and  606 , the inducted airflow range in which engine  28  operating is determined. These ranges are described as range (i), range (j) . . . , range (n) for this example wherein “n” inducted airflow ranges are used to advantage. 
     Assuming engine operation is within airflow range (i), transitions between states of signal FEGO 2 S are counted to generate count signal CF 2   i . This count is compared to maximum count CF 2   imax  during step  612 . While engine operation remains within airflow range (i), a test period of predetermined duration is generated by incrementing count CF 2   i  each transition of signal FEGO 2 S until count CF 2   i  is equal to maximum count CF 2   imax  (step  616 ). During this test period (i), count CR 2   i  is incremented each transition of signal REGO 2 S (step  618 ). Stated another way, count CR 2   i  is incremented each transition of signal REGO 2 S until count CR 2   i =CR 2   imax . 
     When engine operation is within airflow range (j) as shown in step  604 , predetermined period (j), count CF 2   j , and count CR 2   j  are determined in steps  622 ,  626 , and  628  in a manner similar to that described above for airflow range (i) with respect to steps  612 ,  616 , and  618 . Each transition in signal FEGO 2 S, count CF 2   j  is incremented until it reaches maximum count CF 2   jmax  (step  622 ). Predetermined test period (j) is thereby defined. During test period (j), count CR 2   j  is increment each transition of signal REGO 2 S (step  628 ). 
     The above described operation occurs for each airflow range. For example, when engine  28  is operating within airflow range (n) as shown in step  606 , test period (n), count CF 2   n , and count CR 2   n  are generated as shown in steps  632 ,  636 , and  638 . 
     During step  650 , a determination is made as to whether engine  28  has operated in all airflow ranges (i . . . n) for the respective test periods (i . . . n). Stated another way, step  650  determines when each count of transitions in signal FEGO 2 S (CF 2   i , CF 2   j , . . . CF 2   n ) have reached their respective maximum values (CF 2   imax , CF 2   jmax , . . . CF 2   nmax ). 
     Each count (CF 2   i  . . . CF 2   n ) of transitions in signal FEGO 2 S for respective test periods (i . . . n) are summed in step  654  to generate total count CF 2   t . For reasons described above, the same total count CF 2   t  may be obtained by summing each maximum count (CF 2   imax  . . . CF 2   nmax ) for respective test periods (i . . . n). 
     Total count CR 2   t  is generated in step  656  by summing each count (CR 2   i  . . . CR 2   n ) for respective test periods (i . . . n). A ratio of total count CR 2   t  to total count CF 2   t  is then calculated during step  660  and all counts subsequently reset in step  662 . The total efficiency routine is called in step  664 . 
     The actual ratios calculated in step  560  and  660  may are used to provide a measurement of converter efficiencies. Due to the advantages described previously herein, this indication of converter efficiency is accurate over a wider range of converter efficiencies than heretofore possible. 
     The total efficiency routine for determining hen combined catalytic converter efficiency has degraded below a predetermined level is now described with particular reference to FIG.  8 . When RATIO 1  is greater than a first predetermined threshold (Thresh 1 ) or RATIO 2  is greater than the first predetermined threshold (Thresh 1 ) (step  800 ) then a flag is set indicating that the combined catalytic converter efficiency has degraded below a predetermined level (step  801 ). If not, when RATIO 1  is greater than a second predetermined threshold (Thresh 2 ) and RATIO 2  is greater than a third predetermined threshold (Thresh 3 ) (step  802 ) then a flag is set indicating that the combined catalytic converter efficiency has degraded below a predetermined level (step  801 ). If not, when RATIO 2  is greater than the second predetermined threshold (Thresh 2 ) and RATIO 1  is greater than the third predetermined threshold (Thresh 3 ) (step  804 ) then a flag is set indicating that the combined catalytic converter efficiency has degraded below a predetermined level (step  801 ). 
     FIG. 9 shows an example of a curve described by the routine in FIG.  8 . When the combined plot of first and second bank switch ratios (RATIO 1  and RATIO 2 ) fall within the cross-hatched region, the combined catalytic converter efficiency is within acceptable limits. When the combined plot of first and second bank switch ratios (RATIO 1  and RATIO 2 ) fall outside the cross-hatched region, the flag is set indicating that the combined catalytic converter efficiency has degraded below a predetermined level. 
     An example of operation has been presented wherein the routine described in FIG. 8 represents a piece-wise linear curve separating acceptable and unacceptable combinations of first and second bank switch ratios. Alternatively, the routine described in FIG. 8 could be reconfigured by one of ordinary skill in the art to represent any possible two-dimensional curve. The curve could be determined by experimental testing, theoretical models, any combination of the two, or any other method known to those skilled in the art and suggested by this disclosure. Also, the method could be reconfigured so that a function of RATIO 1  and RATIO 2  (for example, an elliptical function described by the function f below) could be compared to a single predetermined number.        f   =         (     RATIO1   A     )     2     +       (     RATIO2   B     )     2                              
     Although one example of an embodiment which practices the invention has been described herein, there are numerous other examples which could also be described. For example, the invention may be used to advantage with carbureted engines, proportional exhaust gas oxygen sensors, and engines having an in-line configuration rather than a V-configuration. The invention is therefore to be defined only in accordance with the following claims.