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A method of assessing overall efficiency of a selective-catalytic-reduction catalyst includes monitoring instantaneous efficiency of the catalyst. The method also includes determining the overall efficiency by summing instantaneous efficiency values weighted by a first set of coefficients if the mos

A method of assessing overall efficiency of a selective-catalytic-reduction catalyst includes monitoring instantaneous efficiency of the catalyst. The method also includes determining the overall efficiency by summing instantaneous efficiency values weighted by a first set of coefficients if the most recent instantaneous efficiency value is above an instantaneous efficiency threshold. The method additionally includes determining the overall efficiency by summing instantaneous efficiency values weighted by a second set of coefficients if the most recent instantaneous efficiency value is equal to or below the instantaneous efficiency threshold. Furthermore, the method includes determining whether the overall efficiency has dropped below an overall efficiency threshold and reporting when the overall efficiency has dropped below the overall efficiency threshold. The second set of coefficients weighs the most recent instantaneous efficiency value more heavily than preceding instantaneous efficiency values as compared with the first set of coefficients. A system and a vehicle are also disclosed.

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1. A method of assessing overall operating efficiency of a selective catalytic reduction (SCR) catalyst, the method comprising: monitoring instantaneous efficiency of the SCR catalyst;determining the overall operating efficiency of the SCR catalyst by summing values of the instantaneous efficiency w

1. A method of assessing overall operating efficiency of a selective catalytic reduction (SCR) catalyst, the method comprising: monitoring instantaneous efficiency of the SCR catalyst;determining the overall operating efficiency of the SCR catalyst by summing values of the instantaneous efficiency weighted by a first set of coefficients if the most recent value of the instantaneous efficiency of the SCR catalyst is above an instantaneous efficiency threshold;determining the overall operating efficiency of the SCR catalyst by summing values of the instantaneous efficiency weighted by a second set of coefficients if the most recent value of the instantaneous efficiency of the SCR catalyst is equal to or below the instantaneous efficiency threshold;determining whether the overall operating efficiency of the SCR catalyst has dropped below an overall operating efficiency threshold; andreporting when the overall operating efficiency of the SCR catalyst has dropped below the overall operating efficiency threshold;wherein the second set of coefficients weighs the most recent instantaneous efficiency value more heavily than preceding instantaneous efficiency values as compared with the first set of coefficients. 2. The method of claim 1, wherein said determining the overall operating efficiency of the SCR catalyst by summing values of the instantaneous efficiency weighted by the second set of coefficients is accomplished for a predetermined number of values of the instantaneous efficiency. 3. The method of claim 2, wherein said determining of the overall operating efficiency of the SCR catalyst includes employing an exponentially-weighted moving average (EWMA) filter to decrease the effect of more recent instantaneous efficiency values on the overall operating efficiency determination. 4. The method of claim 2, wherein the SCR catalyst is employed for reducing oxides of nitrogen (NOX) emissions from an internal combustion engine. 5. The method of claim 4, wherein the engine is a diesel type and the SCR catalyst employs a diesel-exhaust-fluid (DEF) as a reductant supplied from a reservoir for reducing NOX emissions. 6. The method of claim 4, wherein the predetermined number of values of the instantaneous efficiency is collected during a single key cycle of the engine. 7. The method of claim 1, wherein the overall operating efficiency of the SCR catalyst being below the overall operating efficiency threshold is indicative of the SCR catalyst having failed. 8. The method of claim 1, wherein each of said monitoring instantaneous efficiency of the SCR catalyst, determining the overall operating efficiency of the SCR catalyst, determining whether the overall operating efficiency of the SCR catalyst has dropped below the overall operating efficiency threshold, and reporting when the overall operating efficiency of the SCR catalyst has dropped below the overall operating efficiency threshold is accomplished by a controller. 9. The method of claim 1, wherein said reporting when the overall operating efficiency of the SCR catalyst has dropped below the overall operating efficiency threshold is accomplished by generating an alert to an operator of the engine. 10. A system for assessing overall operating efficiency of a selective-catalytic-reduction (SCR) catalyst employed for treating exhaust emissions from an internal combustion engine, the system comprising: a passage configured to direct an exhaust gas stream from the engine to the SCR catalyst;a reservoir configured to supply a reductant through the passage and into the SCR catalyst for reducing oxides of nitrogen (NOX) in the exhaust gas stream;a controller configured to: monitor instantaneous efficiency of the SCR catalyst;determine the overall operating efficiency of the SCR catalyst by summing values of the instantaneous efficiency weighted by a first set of coefficients if the most recent value of the instantaneous efficiency of the SCR catalyst is above an instantaneous efficiency threshold;determine the overall operating efficiency of the SCR catalyst by summing values of the instantaneous efficiency weighted by a second set of coefficients if the most recent value of the instantaneous efficiency of the SCR catalyst is equal to or below the instantaneous efficiency threshold;determine whether the overall operating efficiency of the SCR catalyst has dropped below an overall operating efficiency threshold; andreport when the overall operating efficiency of the SCR catalyst has dropped below the overall operating efficiency threshold;wherein the second set of coefficients weighs the most recent instantaneous efficiency value more heavily than preceding instantaneous efficiency values as compared with the first set of coefficients. 11. The system of claim 10, wherein the controller determines the overall operating efficiency of the SCR catalyst by summing values of the instantaneous efficiency weighted by the second set of coefficients for a predetermined number of values. 12. The system of claim 11, wherein the controller determines the overall operating efficiency of the SCR catalyst by employing an exponentially-weighted moving average (EWMA) filter to decrease the effect of more recent instantaneous efficiency values on the overall operating efficiency determination. 13. The system of claim 10, wherein the controller is configured to report when the overall operating efficiency of the SCR catalyst has dropped below the overall operating efficiency threshold by generating an alert to an operator of the engine. 14. The system of claim 10, wherein the engine is a diesel type and the reductant is a diesel-exhaust-fluid (DEF). 15. The system of claim 10, wherein the predetermined number of values of the instantaneous efficiency is collected during a single key cycle of the engine. 16. The system of claim 10, wherein the overall operating efficiency of the SCR catalyst being below the overall operating efficiency threshold is indicative of the SCR catalyst having failed. 17. A vehicle comprising: an internal combustion engine configured to propel the vehicle;an exhaust passage configured to direct an exhaust gas stream from the engine to a selective-catalytic-reduction (SCR) catalyst;a reservoir configured to supply a reductant through the passage and into the SCR catalyst for reducing oxides of nitrogen (NOX) in the exhaust gas stream; anda controller configured to: monitor instantaneous efficiency of the SCR catalyst;determine an overall operating efficiency of the SCR catalyst by summing values of the instantaneous efficiency weighted by a first set of coefficients if the most recent value of the instantaneous efficiency of the SCR catalyst is above an instantaneous efficiency threshold;determine the overall operating efficiency of the SCR catalyst by summing values of the instantaneous efficiency weighted by a second set of coefficients if the most recent value of the instantaneous efficiency of the SCR catalyst is equal to or below the instantaneous efficiency threshold;determine whether the overall operating efficiency of the SCR catalyst has dropped below an overall operating efficiency threshold; andreport when the overall operating efficiency of the SCR catalyst has dropped below the overall operating efficiency threshold;wherein the second set of coefficients weighs the most recent instantaneous efficiency value more heavily than preceding instantaneous efficiency values as compared with the first set of coefficients; andwherein the overall operating efficiency of the SCR catalyst being below the overall operating efficiency threshold is indicative of the SCR catalyst having failed. 18. The vehicle of claim 17, wherein the controller determines the overall operating efficiency of the SCR catalyst by summing values of the instantaneous efficiency weighted by the second set of coefficients for a predetermined number of values during a single key cycle of the engine. 19. The vehicle of claim 18, wherein the controller determines the overall operating efficiency of the SCR catalyst by employing an exponentially-weighted moving average (EWMA) filter to decrease the effect of more recent instantaneous efficiency values on the overall operating efficiency determination. 20. The vehicle of claim 17, wherein the engine is a diesel type and the reductant is a diesel-exhaust-fluid (DEF).