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A method for accurately diagnosing deterioration in an emission control system for a lean burn internal combustion engine is presented. The emission control system includes an Active Lean NOx (ALNC) Catalyst and a system for injecting a hydrocarbon-based reductant into the ALNC thereby improving its

A method for accurately diagnosing deterioration in an emission control system for a lean burn internal combustion engine is presented. The emission control system includes an Active Lean NOx (ALNC) Catalyst and a system for injecting a hydrocarbon-based reductant into the ALNC thereby improving its NOx conversion efficiency. The method teaches calculating a rate of change of exotherm across the ALNC and differentiating between deterioration of the ALNC and deterioration of the reductant injection system based on the magnitude and the sign of the rate of change of the exotherm. Once the diagnosis of deterioration is made, and the deteriorated component is identified, appropriate corrective measures, such as adjusting or discontinuing the reductant injection amount, and setting diagnostic codes, are taken.

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What is claimed is: 1. A method for diagnosing degradation an emission control system for a lean burn internal combustion engine, the system including an Active Lean NOx Catalyst (ALNC) and a reductant injection system coupled upstream of the ALNC, the method comprising: calculating a rate of chang

What is claimed is: 1. A method for diagnosing degradation an emission control system for a lean burn internal combustion engine, the system including an Active Lean NOx Catalyst (ALNC) and a reductant injection system coupled upstream of the ALNC, the method comprising: calculating a rate of change of an exotherm across the ALNC; and differentiating between the ALNC and the reductant injection system as a cause of the emission control system degradation, where the reductant injection system is identified as said cause when said calculated rate of change of said exotherm is in a first region, and the ALNC is identified as said cause when said calculated rate of change of said exotherm is in a second region less than said first region. 2. The method as set forth in claim 1 wherein said rate of change of said exotherm is determined by differentiating over time a difference between a temperature of an exhaust gas mixture exiting the ALNC and a temperature of an exhaust gas mixture entering the ALNC. 3. The method as set forth in claim 2 wherein said expected rate of change of exotherm is based on engine operating conditions. 4. The method as set forth in claim 3 wherein said engine operating conditions comprise engine speed. 5. The method as set forth in claim 4 wherein said operating conditions further comprise engine load. 6. The method as set forth in claim 5 wherein said operating conditions further comprise catalyst age. 7. The method as set forth in claim 6 wherein said operating conditions further comprise an ALNC temperature. 8. A method for diagnosing degradation an emission control system for a lean burn internal combustion engine, the system including an Active Lean NOx Catalyst (ALNC) and a reductant injection system coupled upstream of the ALNC, the method comprising: calculating a rate of change of an exotherm across the ALNC; differentiating between the ALNC and the reductant injection system as a cause of the emission control system degradation based on said calculated rate of change of exotherm; and providing an indication that the reductant injection system is leaking when said calculated rate of change of exotherm is greater than zero. 9. The method as set forth in claim 8 further comprising adjusting an amount of reductant supply to the reductant injection system in response to said indication to compensate for said leaking. 10. The method as set forth in claim 9 further comprising providing an indication that the reductant injection system is stuck open when said calculated rate of change of exotherm is greater than a predetermined positive value. 11. The method as set forth in claim 10 further comprising turning off reductant supply to the reductant injection system in response to said indication. 12. The method as set forth in claim 11 further comprising setting a diagnostic code. 13. The method as set forth in claim 12 further comprising lighting up an indicator light. 14. A method for diagnosing degradation an emission control system for a lean burn internal combustion engine, the system including an Active Lean NOx Catalyst (ALNC) and a reductant injection system coupled upstream of the ALNC, the method comprising: calculating a rate of change of an exotherm across the ALNC; differentiating between the ALNC and the reductant injection system as a cause of the emission control system degradation based on said calculated rate of change of exotherm; and increasing an amount of reductant injection into the ALNC if said calculated rate of change of exotherm is less than zero and greater than a predetermined negative value. 15. The method as set forth in claim 14 further comprising setting a diagnostic code if said calculated rate of change of exotherm is less than said predetermined negative value. 16. The method as set forth in claim 15 further comprising lighting an indicator light. 17. The method as set forth in claim 16 further comprising turning off reductant supply to the reductant injection system. 18. A method for diagnosing degradation in a reductant injection system adapted to inject a reductant into an exhaust gas aftertreatment device, the method comprising: generating an exotherm across the device by injecting the reductant into the device; calculating a rate of change over an interval of said exotherm; and providing an indication that the reductant injection system is degraded based on said calculated rate of change of said exotherm. 19. The method as set forth in claim 18 wherein the exhaust gas aftertreatment device is an ALNC. 20. The method as set forth in claim 19 wherein the reductant is hydrocarbon. 21. The method as set forth in claim 20 further comprising adjusting an amount of the reductant injected into device in response to said indication of degradation. 22. The method as set forth in claim 18 further comprising discontinuing reductant injection into the device if said calculated rate of change of said exotherm is less than a predetermined negative value. 23. The method as set forth in claim 18 further comprising providing a second indication that the an exhaust gas aftertreatment device is degraded based on said calculated rate of change of said exotherm. 24. A system for monitoring a performance of an internal combustion engine, comprising: an exhaust gas aftertreatment device; an injector adapted to inject a reductant into said device; and a controller for injecting said reductant into said device via said injector thereby creating an exotherm across said device, said controller further calculating a rate of change of said exotherm, and indicating that said injector is degraded when said calculated rate of change of said exotherm is in a first region, and indicating that said exhaust gas aftertreatment device is degraded when said calculated rate of change of said exotherm is in a second region less than said first region. 25. An article of manufacture comprising: a computer storage medium having a computer program encoded therein for use with an emission control system for an internal combustion engine, said emission control system including an exhaust gas aftertreatment device and a reductant injection system injecting a reductant into said exhaust gas aftertreatment device, said computer program comprising: code for providing an indication of an operating condition; code for calculating a rate of change of an exotherm over time across said device in response to said indication; and code for adjusting an amount of said reductant injected into said device based on said calculated rate of change of said exotherm. 26. A method for controlling an emission control system for an internal combustion engine, the system including an exhaust gas aftertreatment device and a reductant injection system coupled upstream of the device, the method comprising: providing an indication of an operating condition; and in response to said indication adjusting an amount of reductant injected into the device based on a rate of change over time of an exotherm generated across the device as a result of a reductant injection to compensate for degradation of one of the reductant delivery system and the exhaust gas aftertreatment device.