Patent ID: 8596042

Claim:
An SCR catalyst control system for an engine having an SCR catalyst, said control system including control hardware comprising: a NO x engine emission determination means for measuring or calculating a NO x engine emission value; a urea control means enabled to supply a quantity of urea to the SCR catalyst; a NO x efficiency target model, which calculates a stored NH 3 target value in the SCR catalyst based on a target conversion NO x efficiency and an SCR catalyst temperature, measured or calculated from an SCR catalyst temperature determination means; an SCR catalyst computational model which calculates a stored NH 3 value in the SCR catalyst based on the NO x engine emission value, the SCR catalyst temperature, the quantity of urea supplied to the SCR catalyst and a pre-determined efficiency of conversion of NO x gases; and characterized by a NO x ratio model which calculates a NO x ratio using a predetermined characteristic of oxidation of NO to NO 2 in an oxidation catalyst as a function of temperature and based on a first temperature value received from a first temperature sensor, the NOx ratio being the ratio of nitrogen dioxide to total NO x in the NO x engine emission value, and wherein the SCR catalyst model takes into account the NO x ratio when calculating the stored NH 3 value, a first subtractor enabled to compare the stored NH 3 target value and the stored NH 3 value in the SCR catalyst to calculate a stored NH 3 differential, wherein the urea control means supplies the required quantity of urea to the SCR catalyst based on the stored NH 3 differential.