Diagnostic methods for selective catalytic reduction (scr) exhaust treatment system

Abstract

In an internal combustion engine system having an exhaust aftertreatment system including a selective catalytic reduction (SCR) catalyst, diagnostic methods involve the intrusive perturbation of a target surface coverage parameter theta to determine the state of health of the SCR catalyst or an ammonia concentration sensor. An adaptive learning block adapts the target theta based on the use of NH 3 sensing feedback from a mid-brick positioned ammonia concentration sensor to pull in system variation. A further diagnostic monitors the amount of adaptation and when the adaptive learning excessively learns, the diagnostic assumes that some system-level degradation must have occurred and the diagnostic will notify the overall emissions control monitor.

1 . In an internal combustion engine having an exhaust treatment system having a selective catalytic reduction (SCR) catalyst, a method of performing a diagnostic on the exhaust treatment system, comprising the steps of:
 introducing a reductant into an exhaust gas stream in an amount based on at least a target surface coverage parameter theta (θ);   perturbing theta in accordance with a diagnostic function;   measuring an operating characteristic of the treatment system;   determining a state of health of a component of the treatment system based on the diagnostic function and the measured operating characteristic.   
   
   
       2 . The method of  claim 1  wherein said reductant is selected from the group comprising ammonia (NH 3 ) and urea, said measuring step including the substep of measuring an ammonia concentration level. 
   
   
       3 . The method of  claim 2  wherein said one component comprises the SCR catalyst, said state of health including an ammonia storage capability of the SCR catalyst, said step of determining the state of health including the sub-steps of:
 comparing the measured ammonia concentration level with a predetermined threshold;   determining a state of health fault based on an amount that the measured ammonia concentration exceeds the predetermined threshold.   
   
   
       4 . The method of  claim 3  further including the step of setting an SCR catalyst fault. 
   
   
       5 . The method of  claim 3  wherein said diagnostic function comprises one selected from the group (including a periodic function and a non-periodic function) a periodic function, said step of determining the state of health further includes the substep of determining whether the measured ammonia concentration level correlates to the diagnostic function. 
   
   
       6 . The method of  claim 3  further comprising the steps of:
 providing an ammonia concentration sensor for measuring ammonia concentration; and   verifying proper operation of the ammonia concentration sensor.   
   
   
       7 . The method of  claim 6  further comprising the step of:
 positioning the ammonia concentration sensor in a sensing location selected from the group comprising a mid-brick position of the SCR catalyst and a rear-brick position of the SCR catalyst.   
   
   
       8 . The method of  claim 5  wherein said theta parameter is controlled in accordance with a control strategy configured to increase NOx conversion in the SCR catalyst and reduce NH 3  emission from the SCR catalyst, said diagnostic function being configured to result in a detectable excess of NH 3  emission from the SCR catalyst. 
   
   
       9 . The method of  claim 2  wherein said determining a state of health step includes the sub-step of:
 comparing an aspect of the measured operating characteristic and the diagnostic function wherein the aspect is selected from the group comprising (i) a signal amplitude; (ii) a phase or time delay; and (iii) a frequency difference.   
   
   
       10 . The method of  claim 2  wherein said one component comprises an ammonia concentration sensor, said diagnostic function comprising a periodic function, said step of determining the state of health including the sub-steps of:
 comparing a correlation factor by comparing the measured ammonia concentration level with periodic function;   determining a state of health fault based on the correlation factor.   
   
   
       11 . The method of  claim 10  further including the step of setting an ammonia concentration sensor fault. 
   
   
       12 . The method of  claim 11  further comprising the step of:
 positioning the ammonia concentration sensor in a sensing location selected from the group comprising a mid-brick position of the SCR catalyst and a rear-brick position of the SCR catalyst.   
   
   
       13 . In an internal combustion engine having an exhaust treatment system having a selective catalytic reduction (SCR) catalyst, a method of performing a diagnostic on the exhaust treatment system, comprising the steps of:
 introducing a reductant into an exhaust gas stream in an amount based on at least a surface coverage parameter theta ( 0 ) selected so as to increase NOx conversion and reduce NH 3  emission from the SCR catalyst;   adapting the theta parameter based on a measured NH 3  level emitted from the SCR catalyst;   generating an exhaust system fault when an adaptation amount for the theta parameter exceeds a predetermined threshold.   
   
   
       14 . The method of  claim 13  wherein said generating step includes the sub-steps of:
 establishing respective upper and lower adaptation limits;   generating the fault when the adaptations of the theta parameter exceeds one of the upper and lower adaptation limits.   
   
   
       15 . The method of  claim 14  wherein said reductant comprises one of ammonia and aqueous urea, said step adapting step includes the sub-steps of:
 defining a base value for the theta parameter based on an inlet temperature of the SCR catalyst;   determining a compensation factor based on a measured NH 3  concentration level emitted from the SCR catalyst; and   determining the adapted theta parameter value in accordance with the base value and the compensation factor.   
   
   
       16 . The method of  claim 15  wherein said sub-step of determining a compensation factor includes the sub-step of:
 determining when the measured NH 3  concentration level exceeds an upper bound and increasing the compensation factor.   
   
   
       17 . The method of  claim 16  wherein said sub-step of determining a compensation factor includes the sub-step of:
 determining when the measured NH 3  concentration level is less than a lower bound and decreasing the compensation factor.