This application is a continuation of Ser. No. 09/488,419 filed Jan. 20, 2000. 
    
    
     TECHNICAL FIELD 
     The present invention is directed to a system for monitoring catalyst operation in an internal combustion engine having a two-bank exhaust system. More particularly, the invention is directed to a diagnostic system that monitors catalyst efficiency by comparing signals between a pre-catalyst EGO sensor and a post-catalyst EGO sensor in two different banks. 
     BACKGROUND ART 
     To meet current emission regulations, automotive vehicles must regulate the air/fuel ratio (A/F) supplied to the vehicles&#39; cylinders so as to achieve maximum efficiency of the vehicles&#39; catalysts. For this purpose, it is known to control the air/fuel ratio of internal combustion engines using an exhaust gas oxygen (EGO) sensor positioned in the exhaust stream from the engine. The EGO sensor provides feedback data to an electronic controller that calculates preferred A/F values over time to achieve optimum efficiency of a catalyst in the exhaust system. More particularly, the EGO sensor feedback signals are used to calculate desired A/F ratios via a jumpback and ramp process, which is known in the art. 
     It is also known to have systems with two EGO sensors in a single exhaust stream in an effort to achieve more precise A/F control with respect to the catalyst window. Normally, a pre-catalyst EGO sensor is positioned upstream of the catalyst and a post-catalyst EGO sensor is positioned downstream of the catalyst. Finally, in connection with engines having two groups of cylinders, it is known to have a two-bank exhaust system coupled thereto where each exhaust bank has its own catalyst as well as its own pre-catalyst and post-catalyst EGO sensors. 
     It is known in the art to monitor the efficiency of a catalyst by determining the arc length ratio between signals generated by corresponding pre-catalyst and post-catalyst EGO sensors in the same exhaust stream and connected to the same catalyst. This type of system is described in U.S. Pat. No. 5,899,062 to Jerger et al. and entitled “Catalyst Monitor Using Arc Length Ratio of Pre- and Post-Catalyst Signals”, the disclosure of which is incorporated herein by reference. 
     Sometimes, in a two-bank, four-EGO sensor exhaust system, one of the pre-catalyst EGO sensors degrades. In other circumstances, it is desirable to purposely eliminate one of the pre-catalyst EGO sensors in a two-bank system to reduce the cost of the system. In either event, it is desirable to be able to monitor the catalyst efficiency in the group of cylinders coupled to the exhaust bank having only one operational EGO sensor by using the signals received from the three operational EGO sensors alone. However, known methods for catalyst diagnosis require a matched set of pre-catalyst and post-catalyst EGO sensors in each bank, such as in a one-bank, two EGO sensor system or in a two-bank, four EGO sensor system, so that the arc lengths between the corresponding pre-catalyst and post-catalyst sensors can be compared. Thus, for a two-bank, three EGO sensor system, only the catalyst in the two EGO sensor exhaust bank will be monitored and diagnosed, while the catalyst in the bank having only one operational EGO sensor will remain unmonitored. 
     There is a need for an improved system that can monitor the operation of a catalyst in a one-sensor bank even though the catalyst only has one EGO sensor coupled to it. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed toward a new system and method for monitoring the operation of both catalysts in an internal combustion engine having a group of cylinders coupled to two functioning EGO sensors (the “two-sensor bank”) and another group of cylinders coupled to one functioning EGO sensor (the “one-sensor bank”). More particularly, the operation of the catalyst in the one-sensor bank is monitored and diagnosed based on a signal from a post-catalyst EGO sensor connected to the catalyst and a signal from a pre-catalyst EGO sensor in a different bank and connected to a different catalyst. 
     In a preferred embodiment of the invention, for a system that is a missing a pre-catalyst EGO sensor in the one-sensor bank, the signal from the pre-catalyst EGO sensor in the two-sensor bank is used to calculate a diagnostic signal for the catalyst in the one-sensor bank. In essence, the invention assumes that a signal characteristic for the non-existent pre-catalyst EGO sensor in the one-sensor bank would be the same as the signal characteristic of the existing pre-catalyst EGO sensor in the two-sensor bank and calculates a diagnostic signal for the catalyst in the one-sensor bank accordingly. The diagnostic signal can be, for example, a ratio of the arc lengths between the post-catalyst and pre-catalyst EGO sensor signals. 
     Once the arc length ratios are calculated, the ratios can be compared with calibratable or experimentally-generated ratios to monitor the catalyst efficiency over time. As a result, the invention can monitor and diagnose the operation of the catalysts in both the one-sensor bank and the two-sensor bank even though the one-sensor bank does not have a matched pair of EGO sensors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an internal combustion engine according to a preferred embodiment of the invention; 
     FIG. 2 is a block diagram representing a known two-bank exhaust system with each bank having pre-catalyst and post-catalyst EGO sensors; 
     FIG. 3 is a flowchart illustrating a known method in which the arc length ratio is calculated for a two-sensor bank. 
     FIG. 4 is a block diagram representing a two-bank exhaust system wherein one bank has a pre-catalyst and a post-catalyst EGO sensor and the other bank has only a post-catalyst EGO sensor; and 
     FIG. 5 is a flowchart illustrating the inventive method in which the arc length ratio is calculated. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates an internal combustion engine. Engine  200  generally comprises a plurality of cylinders, but, for illustration purposes, only one cylinder is shown in FIG.  1 . Engine  200  includes combustion chamber  206  and cylinder walls  208  with piston  210  positioned therein and connected to crankshaft  212 . Combustion chamber  206  is shown communicating with intake manifold  214  and exhaust manifold  216  via respective intake valve  218  and exhaust valve  220 . As described later herein, engine  200  may include multiple exhaust manifolds with each exhaust manifold corresponding to a group of engine cylinders. Intake manifold  214  is also shown having fuel injector  226  coupled thereto for delivering liquid fuel in proportion to the pulse width of signal FPW from controller  202 . Fuel is delivered to fuel injector  226  by a conventional fuel system (not shown) including a fuel tank, fuel pump, and fuel rail (not shown). 
     Conventional distributorless ignition system  228  provides ignition spark to combustion chamber  206  via spark plug  230  in response to controller  202 . A first two-state EGO sensor  204  is shown coupled to exhaust manifold  216  upstream of catalyst  232 . A second two-state EGO sensor  234  is shown coupled to exhaust manifold  216  downstream of catalyst  232 . The upstream EGO sensor  204  provides a feedback signal EGO 1  to controller  202  which converts signal EGO 1  into two-state signal EGOS 1 . A high voltage state of signal EGOS 1  indicates exhaust gases are rich of a reference A/F and a low voltage state of converted signal EGO 1  indicates exhaust gases are lean of the reference A/F. The downstream EGO sensor  234  provides signal EGO 2  to controller  202  which converts signal EGO 2  into two-state signal EGOS 2 . A high voltage state of signal EGOS 2  indicates that the engine is running rich, and a low voltage state of converted signal EGO 1  indicates that the engine is running lean. Controller  202  is shown in FIG. 1 as a conventional microcomputer including: microprocessor unit  238 , input/output ports  242 , read only memory  236 , random access memory  240 , keep alive memory  241  and a conventional data bus. 
     FIGS. 2 and 4 schematically illustrate different embodiments of a two-bank exhaust system to be used in the present invention. FIG. 2 shows a known two-bank, four EGO-sensor exhaust system. As illustrated in FIG. 2, exhaust gases flow from first and second groups of cylinders of engine  12  through a corresponding first exhaust bank  14  and second exhaust bank  16 . Engine  12  is the same as or similar to engine  200  in FIG.  1 . Exhaust bank  14  includes pre-catalyst EGO sensor  18 , catalyst  20 , and post-catalyst EGO sensor  22 . Exhaust bank  16  includes pre-catalyst EGO sensor  24 , catalyst  26  and post-catalyst EGO sensor  28 . The pre-catalyst EGO sensors, catalysts, and post-catalyst EGO sensors in FIG. 2 are the same as or similar to pre-catalyst EGO sensor  204 , catalyst  232 , and post-catalyst EGO sensor  234  in FIG.  1 . 
     In operation, when exhaust gases flow from engine  12  through exhaust bank  14 , pre-catalyst EGO sensor  18  senses the emissions level in the exhaust gases passing through bank  14  before they enter catalyst  20  and provides feedback signal EGO 1   a  to controller  202 . After the exhaust gases pass through catalyst  20 , post-catalyst EGO sensor  22  senses the emissions level in the exhaust gases after they exit the catalyst  20  and provides feedback signal EGO 1   b  to controller  202 . With respect to exhaust bank  16 , pre-catalyst EGO sensor  24  senses the emissions level in the exhaust gases passing through bank  16  before they enter catalyst  26  and provides feedback signal EGO 2   a  to controller  202 . After the exhaust gases pass through catalyst  26 , post-catalyst EGO sensor  28  senses the emissions level in the exhaust gases after they exit catalyst  26  and provides feedback signal EGO 2   b  to controller  202 . Then the exhaust gases are joined at junction  29  before being expelled from the system  10 , though the disclosed invention is equally applicable to a system wherein the exhaust banks are kept separate throughout the entire system. Controller  202  uses feedback signals EGO 1   a,  EGO 1   b,  EGO 2   a,  and EGO 2   b,  which reflect the current operating conditions of the catalysts  20 ,  26 , to calculate the arc length ratios for diagnosing catalyst operation. The controller shown in FIG. 2 is the same as or similar to controller  202  shown in FIG.  1 . 
     Catalyst operation can be monitored by comparing selected signal characteristics, such as the arc length, of the signals from the pre-catalyst and post-catalyst EGO sensors connected to that catalyst. Although the present application focuses on calculating a catalyst diagnostic signal based on the arc lengths of the EGO sensor signals, any signal characteristic can be used as long as one signal is from a pre-catalyst EGO sensor and the other signal is from a post-catalyst EGO sensor, even if the sensors are in different exhaust banks. One way in which the arc length ratios are calculated for a two-sensor bank is explained in U.S. Pat. No. 5,899,062, which is incorporated herein by reference. A flowchart of the known calculation process is shown in FIG.  3 . Because each catalyst  20 ,  26  is coupled to both a pre-catalyst EGO sensor  18 ,  22  and a post-catalyst EGO sensor  24 ,  28  in each bank  14 ,  16 , the same process is used to calculate the arc length ratios for monitoring each catalyst  20 ,  26 . In this case, the system samples both the pre-catalyst EGO sensor signals and post-catalyst EGO sensor signals  32  and then determines incremental signal arc lengths  34  from the samples. An instantaneous ratio is calculated  36  from the incremental arc lengths, preferably by dividing the incremental arc length of the post-catalyst signal by the incremental arc length of the pre-catalyst signal for a given catalyst. The system then sums the incremental arc lengths of each signal  38  from the EGO sensors to obtain an estimate of the line integral for a particular signal segment and calculates an accumulated arc length ratio based on the summed arc lengths  40 . The instantaneous and accumulated arc length ratios are then stored in memory  42  and used to monitor the efficiency of the catalyst  44 . For example, the arc length of the post-catalyst signal with respect to the arc length of the pre-catalyst signal will increase as the catalyst ages and becomes less efficient. 
     FIG. 4 illustrates a two-bank exhaust system similar to that shown in FIG. 2, except that the pre-catalyst EGO sensor in one of the exhaust banks  36  is missing. Specifically, FIG. 4 illustrates that exhaust gases expelled from engine  32  pass through exhaust banks  34  and  36 . In bank  34 , the emissions level of the exhaust gases is sensed by pre-catalyst EGO sensor  38  before entering catalyst  40 , and feedback signal EGO 1   a  is provided to controller  202 . After the exhaust gases exit catalyst  40 , the emissions level is sensed by post-catalyst EGO sensor  42 , and feedback signal EGO 2   a  is provided to controller  202 . With respect to exhaust bank  36 , the exhaust gases expelled by engine  32  enter catalyst  44 . After the exhaust gases exit catalyst  44 , their oxygen content is sensed by post-catalyst EGO sensor  46 , and feedback signal EGO 2   b  is provided to controller  202 . Then the exhaust gases are joined at junction  48  before being expelled from the system  30 , though the disclosed invention is equally applicable to a system wherein the exhaust banks are kept separate throughout the entire system. 
     FIG. 5 is a flowchart illustrating the arc length ratio calculation process  50  according to the present invention. Because one of the banks  36  does not have a pre-catalyst EGO sensor, the process must also include the step of checking whether a pre-catalyst sensor is connected to the catalyst being monitored  56 . If both pre-catalyst and post-catalyst EGO sensors are coupled to the catalyst (i.e. the catalyst is in a two-sensor bank), then the system continues calculating the arc length ratio in the known manner explained above  60 ,  62 ,  64 ,  66 . If, however, the catalyst only has a post-catalyst EGO sensor coupled to it with no corresponding pre-catalyst EGO sensor (i.e. the catalyst is in a one-sensor bank, as shown in FIG.  4 ), the invention uses the arc length of the pre-catalyst sensor signal in the two-sensor bank of the engine for the arc ratio calculation in the one-sensor bank  58 . In short, the invention assumes that the arc length of the missing pre-catalyst EGO sensor in the one-sensor bank would be the same as the arc length of the existing pre-catalyst EGO sensor in the two-sensor bank. This allows calculation of the arc ratios for both catalysts with only three measured arc lengths instead of the four arc lengths that are conventionally required in known methods. The arc length ratio calculations according to the present invention would therefore be as follows: 
     
       
         Arc_ratio — 1=Arc_length12/Arc_length11  
       
     
     
       
         Arc_ratio — 2=Arc_length22/Arc_length11  
       
     
     where: 
     Arc_ratio — 1: arc ratio, two-sensor bank 
     Arc_ratio — 2: arc ratio, one-sensor bank 
     Arc_length11: pre-catalyst sensor signal arc length, two sensor bank 
     Arc_length12: post-catalyst sensor signal arc length, two sensor bank 
     Arc_length22: post-catalyst sensor signal arc length, one sensor bank 
     Note that although the present invention was described in terms of a two-bank, three-EGO sensor system, as shown in FIG. 4, it is contemplated and should be understood that this invention can also be used in connection with a well-known two-bank four-EGO sensor system, as shown in FIG. 2, for purposes of compensating for a degraded pre-catalyst EGO sensor in one of the banks. In such a system, known methods, such as the method described in U.S. Pat. No. 5,899,062, can be used to monitor the catalysts in both banks while all four EGO sensors are operating properly. In the event that one of the pre-catalyst EGO sensors degrades, and if the degradation is detected by the system, the invention compensates for the degraded EGO sensors by conducting the arc ratio calculation using only three arc length measurements. 
     It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby.
A method and system for diagnosing catalyst operation in an internal combustion engine having a two-bank, three EGO sensor structure includes determining the ratio of the arc length between the post-catalyst EGO sensor signal and the arc length of a pre-catalyst EGO sensor signal over a selected time period. If the exhaust bank is a one-sensor bank having only a post-catalyst EGO sensor and no pre-catalyst EGO sensor, the system uses the arc length from the pre-catalyst EGO sensor in the two-sensor bank to calculate the arc length ratio, thereby allowing calculation of two arc length ratios without two matched pairs of EGO sensors. The ratio indicates the efficiency of the catalyst and may be compared with calibratable or experimentally-determined thresholds to monitor converter efficiency over time.