Patent ID: 8074445

Claim:
An apparatus for reducing NO x emissions in an engine exhaust gas stream flowing through a selective catalytic reduction (SCR) system having an SCR catalyst positioned downstream of a urea injector operable to inject urea into the exhaust gas stream, the urea being convertible to ammonia and isocyanic acid, comprising: a NO x reduction target module operable to determine a NO x reduction requirement, the NO x reduction requirement comprising an amount of NO x in the exhaust gas stream to be reduced on the SCR catalyst in the presence of ammonia; an ammonia target module operable to determine an ammonia addition requirement, the ammonia addition requirement comprising an amount of ammonia added to the exhaust gas stream to achieve the NO x reduction requirement; a reductant target module comprising: a hydrolysis module operable to determine a predicted amount of ammonia and isocyanic acid entering the SCR catalyst based at least partially on (i) the distance between the SCR catalyst and the urea injector, (ii) a conversion efficiency of urea to ammonia and (iii) a conversion efficiency of urea to isocyanic acid; and an inverse hydrolysis module operable to determine a reductant injection requirement based at least partially on the predicted amount of ammonia and isocyanic acid entering the SCR catalyst, the reductant injection requirement comprising an amount of urea added to the exhaust gas stream to achieve the ammonia addition requirement; wherein the NOx reduction target module, ammonia target module, and reductant target module comprise one or more of logic hardware and executable code, the executable code being stored on one or more non-transitory machine-readable storage media.