Diagnostic system for monitoring catalyst operation using arc length ratio

Abstract: A method and system for diagnosing catalyst operation in an internal combustion engine having a two-bank (34,36), three EGO sensor structure (38,42,46) includes determining the ratio of the arc length between the post-catalyst EGO sensor (46) signal and the arc length of a pre-catalyst EGO sensor (38) signal over a selected time period. In the system embodying the invention when an exhaust bank (36) is a one-sensor bank having only a post-catalyst EGO sensor (46) and no pre-catalyst EGO sensor, the system uses the arc length from the pre-catalyst EGO sensor (38) in the two-sensor bank (34) 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.

Description: 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. To meet current emission regulations, automotive vehicles must regulate the air/fuel ratio (A/F) supplied to the vehicles' cylinders so as to achieve maximum efficiency of the vehicles' 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, as described for example in U.S. Patent 5,261,230 to Yuuki et al. dated 16 November 1993 and entitled "Device for Controlling Heating of Catalyst for Purifying Exhaust Gas". 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. Patent 5,899,062 to Jerger et al. and entitled "Catalyst Monitor Using Arc Length Ratio of Pre- and Post-Catalyst Signals". 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. According to the present invention there is provided a system for monitoring catalyst operation in an engine having a two-bank exhaust system, a first catalyst included in a first exhaust bank and a second catalyst included in a second exhaust bank, comprising: a first EGO sensor coupled to the first catalyst at a post-catalyst location, the first EGO sensor generating a first signal; a second EGO sensor coupled to the second catalyst at a pre-catalyst location, the second EGO sensor generating a second signal; a controller with means for calculating a first diagnostic signal corresponding to the operation of the first catalyst characterised in that the controller is coupled to said first and second EGO sensors and calculates said first diagnostic signal from the arc lengths of the first and second signals. Further according to the present invention there is provided a method for monitoring catalyst operation in an engine having a first catalyst and a second catalyst, comprising: generating a first signal from a first EGO sensor coupled to the first catalyst at a post-catalyst location; generating a second signal from a second EGO sensor coupled to a second catalyst at a pre-catalyst location; calculating at least a first diagnostic signal corresponding to the operation of the first catalyst from the first and second signals. 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. The present invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates an internal combustion engine according to a preferred embodiment of the invention; Figure 2 is a block diagram representing a two-bank exhaust system with each bank having pre-catalyst and post-catalyst EGO sensors; Figure 3 is a flowchart illustrating a method in which the arc length ratio is calculated for a two-sensor bank. Figure 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 Figure 5 is a flowchart illustrating the inventive method in which the arc length ratio is calculated. Figure 1 illustrates an internal combustion engine. Engine 200 generally comprises a plurality of cylinders, but, for illustration purposes, only one cylinder is shown in Figure 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 EGO1 to controller 202 which converts signal EGO1 into two-state signal EGOS1. A high voltage state of signal EGOS1 indicates exhaust gases are rich of a reference A/F and a low voltage state of converted signal EGO1 indicates exhaust gases are lean of the reference A/F. The downstream EGO sensor 234 provides signal EGO2 to controller 202 which converts signal EGO2 into two-state signal EGOS2. A high voltage state of signal EGOS2 indicates that the engine is running rich, and a low voltage state of converted signal EGO1 indicates that the engine is running lean. Controller 202 is shown in Figure 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. Figures 2 and 4 schematically illustrate different embodiments of a two-bank exhaust system to be used in the present invention. Figure 2 shows a two-bank, four EGO-sensor exhaust system. As illustrated in Figure 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 Figure 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 Figure 2 are the same as or similar to pre-catalyst EGO sensor 204, catalyst 232, and post-catalyst EGO sensor 234 in Figure 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 EGO1a 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 EGO1b 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 EGO2a 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 EGO2b 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 EGO1a, EGO1b, EGO2a, and EGO2b, 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 Figure 2 is the same as or similar to controller 202 shown in Figure 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. Patent No. 5,899,062. A flowchart of the known calculation process is shown in Figure 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. Figure 4 illustrates a two-bank exhaust system similar to that shown in Figure 2, except that the pre-catalyst EGO sensor in one of the exhaust banks 36 is missing. Specifically, Figure 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 EGO1a 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 EGO2a 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 EGO2b 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. Figure 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 Figure 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 Figure 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 Figure 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. Patent 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.

Claims: A system for monitoring catalyst operation in an engine (12) having a two-bank exhaust system, a first catalyst (44) included in a first exhaust bank (36) and a second catalyst (40) included in a second exhaust bank (34), comprising: a first EGO sensor (46) coupled to the first catalyst (44) at a post-catalyst location, the first EGO sensor (46) generating a first signal; a second EGO sensor (38) coupled to the second catalyst (40) at a pre-catalyst location, the second EGO sensor (38) generating a second signal; a controller (202) with means for calculating a first diagnostic signal corresponding to the operation of the first catalyst (44); characterised in that the controller (202) is coupled to said first (46) and second (38) EGO sensors and calculates said first diagnostic signal from the arc lengths of the first and second signals. A system as claimed in claim 1, wherein the first signal has a first arc length, the second signal has a second arc length, and the first diagnostic signal is a first arc length ratio of the first and second arc lengths. A system as claimed in claim 2, wherein the first arc length ratio for the first catalyst is the first arc length divided by the second arc length. A system as claimed in claim 2, further comprising a third EGO sensor coupled to the second catalyst at a post-catalyst location, the third EGO sensor generating a third signal, wherein the controller calculates a second diagnostic signal for the second catalyst from the second and third signals. A system as claimed in claim 4, wherein the third signal has a third arc length and the second diagnostic signal is a second arc length ratio of the second and third arc lengths. A system as claimed in claim 5, wherein the first arc length ratio for the first catalyst is the first arc length divided by the second arc length, and wherein the second arc length ratio for the second catalyst is the third arc length divided by the second arc length. A method for monitoring catalyst operation in an engine (12) having a two-bank exhaust system, a first catalyst (44) included in a first exhaust bank (36) and a second catalyst (40) included in a second exhaust bank (34), comprising: generating a first signal from a first EGO sensor (46) coupled to the first catalyst (44) at a post-catalyst location; generating a second signal from a second EGO sensor (38) coupled to the second catalyst (40) at a pre-catalyst location; and calculating at least a first diagnostic signal corresponding to the operation of the first catalyst from the first and second signals. A method as claimed in claim 7, wherein the calculating step comprises the steps of: determining first signal and second signal incremental arc lengths of the first and second signals, respectively; and calculating a first instantaneous arc length ratio from the first and second signal incremental arc lengths obtained in the determining step. A method as claimed in claim 8, wherein the first instantaneous arc length ratio for the first catalyst is the first signal incremental arc length divided by the second incremental signal arc length. A method as claimed in claim 9, wherein the calculating step further comprises the steps of: summing the first and second signal incremental arc lengths of the first and second signals to obtain first and second accumulated arc lengths, respectively; and calculating a first accumulated arc length ratio corresponding to the first catalyst from the first and second accumulated arc lengths. A method as claimed in claim 10, wherein the first accumulated arc length ratio for the first catalyst is the first signal accumulated arc length divided by the second accumulated signal arc length. A method as claimed in claim 7, further comprising the steps of: generating a third signal from a third EGO sensor coupled to the second catalyst at a post-catalyst location, and calculating at least a second diagnostic signal corresponding to the operation of the second catalyst from the second and third signals. A method as claimed in claim 12, wherein the first, second, and third signals have first, second and third arc lengths, respectively, and wherein the first diagnostic signal is a first arc length ratio of the first and second arc lengths and the second diagnostic signal is a second arc length ratio of the second and chird arc lengths. A method as claimed in claim 12, wherein the arc length ratio for the first catalyst is the first arc length divided by the second arc length, and wherein the arc length ratio for the second catalyst is the third arc length divided by the second arc length. A method as claimed in claim 12, wherein the calculating step comprises the steps of: determining first signal, second signal and third signal incremental arc lengths of the first, second and third signals, respectively; calculating a first instantaneous arc length ratio corresponding to the first catalyst from the first and second signal incremental arc lengths obtained in the determining step; and calculating a second instantaneous arc length ratio corresponding to the second catalyst from the second and third incremental arc lengths obtained in the determining step. A method as claimed in claim 15, wherein the first instantaneous arc length ratio for the first catalyst is the first signal incremental arc length divided by the second incremental signal arc length, and wherein the second instantaneous arc length ratio for the second catalyst is the third incremental arc length divided by the second incremental arc length. A method as claimed in claim 16, wherein the calculating step further comprises the steps of: summing the first, second and third signal incremental arc lengths of the first, second and third signals to obtain first, second and third accumulated arc lengths, respectively; calculating a first accumulated arc length ratio corresponding to the first catalyst from the first and second accumulated arc lengths; and calculating a second accumulated arc length ratio corresponding to the second catalyst from the second and third accumulated arc lengths. A method as claimed in claim 17, wherein the first accumulated arc length ratio for the first catalyst is the first signal accumulated arc length divided by the second accumulated signal arc length, and wherein the second accumulated arc length ratio for the second catalyst is the third accumulated arc length divided by the second accumulated arc length.

Inventors: BEHR KENNETH JOHN, BOOTH RICHARD ANDREW, CULLEN MICHAEL JOHN, SEALY BRENT EDWARD, BEHR, KENNETH JOHN, BOOTH, RICHARD ANDREW, CULLEN, MICHAEL JOHN, SEALY, BRENT EDWARD
Applicants: FORD GLOBAL TECH INC, FORD GLOBAL TECHNOLOGIES, INC.
Application year: 2001
Main classification: F01N11
IPC codes: F01N11