Patent ID: 8881508

Claim:
A method, comprising: receiving a portion of an exhaust gas stream into an exhaust outlet flow path, wherein the portion of the exhaust gas stream is removed downstream of a first selective catalytic reduction (SCR) catalyst and upstream of a second SCR catalyst that receives a second portion of the exhaust gas stream; treating the removed portion with a diagnostic SCR catalyst element; determining a composition of the treated removed portion, wherein the determining comprises determining an NH 3 concentration; determining an actuator response function in response to at least one operating condition of the first SCR catalyst, the actuator response function comprising a reductant injector response as a function of the amount of NH 3 ; determining a reductant injection amount in response to the amount of NH 3 and the actuator response function; and injecting an amount of reductant upstream of the first SCR catalyst in response to the reductant injection amount.