Exhaust gas treatment system and methods for operating the same

Abstract

An exhaust gas treatment system includes a selective catalytic reduction (SCR) catalyst and a dosing control responsive to exhaust gas operating conditions for controlling the dosing rate of a reductant such as aqueous urea into the exhaust stream. The dosing control is configured to reduce the dosing rate when either a sudden increase in the exhaust mass air flow is detected or when an exhaust gas temperature gradient is in an increasing state. The dosing control is also configured to shut-off dosing when a measured ammonia concentration level exceeds an ammonia slip trip level, provided that the exhaust gas temperature gradient is also in an increasing state.

1 . In an internal combustion engine producing an exhaust gas stream to an exhaust treatment system having a selective catalytic reduction (SCR) catalyst, a method of reductant slip control, comprising the steps of:
 dosing reductant into the exhaust gas stream;   establishing a reductant slip trip level based on an exhaust gas temperature;   decreasing the reductant dosing when an exhaust gas temperature gradient is in an increasing state and a reductant concentration level measured at the SCR catalyst exceeds the reductant slip trip level.   
   
   
       2 . The method of  claim 1  wherein said step of discontinuing is further performed when an exhaust gas temperature exceeds a predetermined threshold. 
   
   
       3 . The method of  claim 2  wherein said reductant is selected from the group comprising ammonia (NH 3 ) and urea, said reductant concentration level being an ammonia concentration level, said dosing step including the sub-step of mixing the reductant with the exhaust gas upstream of the SCR catalyst. 
   
   
       4 . The method of  claim 3  wherein the SCR catalyst is multi-brick in construction, said method further comprising the step of:
 disposing an ammonia gas concentration sensor at a mid-brick position of the SCR catalyst.   
   
   
       5 . The method of  claim 4  wherein the mid-brick position is located at the substantial center of the SCR catalyst. 
   
   
       6 . The method of  claim 1  where said decreasing step includes the sub-step of discontinuing reductant dosing. 
   
   
       7 . In an internal combustion engine producing a stream of an exhaust gas to an exhaust treatment system having a selective catalytic reduction (SCR) catalyst, a method of operating the treatment system, comprising the steps of:
 dosing reductant into the exhaust gas stream in an amount based on at least a reductant surface coverage parameter theta (θ) of the SCR;   decreasing the reductant dosing when one of a plurality of transient compensation trigger conditions are satisfied, wherein the trigger conditions include a first condition when a rate of change in a mass air flow (MAF) level exceeds a first predetermined threshold and a second condition when an exhaust gas temperature gradient is in an increasing state.   
   
   
       8 . The method of  claim 7  wherein said reductant is selected from the group comprising ammonia (NH 3 ) and urea, said reductant concentration level being an ammonia concentration level, said dosing step including the sub-step of mixing the reductant with the exhaust gas upstream of the SCR catalyst. 
   
   
       9 . The method of  claim 7  wherein said dosing step includes the sub-steps of:
 measuring an exhaust gas temperature;   determining a value for the surface coverage parameter theta (θ) based on measured exhaust gas temperature and predetermined data;   and wherein said decreasing step includes adjusting the determined theta (θ) parameter value downwards by a predetermined amount.   
   
   
       10 . The method of  claim 9  further including the step of:
 increasing the adjusted theta (θ) parameter value when none of the transient compensation triggers conditions are satisfied.   
   
   
       11 . The method of  claim 10  further including the step of:
 repeating said increasing step until the adjusted theta (θ) parameter value equals the theta (θ) parameter value determined based on the measured exhaust gas temperature and the predetermined data.   
   
   
       12 . The method of  claim 7  further including the steps of:
 detecting the first condition at a first time; and   sustaining the first condition for a predetermined time after the first time.   
   
   
       13 . The method of  claim 12  wherein said detection step includes the sub-steps of:
 determining, at an initial time, that a rate of change of the MAF level exceeds the first predetermined threshold; and   deeming the first condition detected when the rate of change of the MAF level continues to exceed the first predetermined threshold as assessed at a confirmation time interval after the initial time.   
   
   
       14 . The method of  claim 7  further including the steps of:
 providing an exhaust gas temperature gradient signal;   establishing predetermined upper and lower state limits;   determining the state of the exhaust gas temperature gradient as (i) increasing when the exhaust gas temperature gradient signal is greater than the upper state limit; (ii) steady state when the exhaust gas temperature gradient signal is between the upper and lower state limits; and (iii) decreasing when the exhaust gas temperature gradient signal is lower than the lower state limit; and   deeming the second condition as detected when the exhaust gas temperature gradient is in the increasing state.