Patent ID: 7200990

Claim:
A method for controlling injection of a reductant into an NO x containing exhaust gas stream from a combustion engine, where the NO x is reduced by selective catalytic reduction, SCR, in the presence of a SCR catalyst, comprising the steps of establishing a signal related to exhaust gas flow; establishing a signal related to NO x concentration in the exhaust gas stream upstream of the SCR catalyst; establishing a signal related to exhaust gas temperature upstream of the SCR catalyst; establishing a signal related to exhaust gas temperature downstream of the SCR catalyst; calculating a molar NO x flow in the exhaust gas stream upstream of the SCR catalyst and establishing a signal related to the molar NO x flow; calculating an NO x conversion in the catalyst and establishing a signal related to the NO x conversion; calculation of a required amount of reductant and establishing a signal related to the required amount of reductant; adjusting the required amount of reductant; establishing a signal related to an adjusted amount of reductant; using the signal related to the adjusted amount of reductant for metering the adjusted amount of reductant; and injecting the adjusted amount of reductant into the exhaust gas; wherein, the required amount of reductant is adjusted by calculation of a value d(E*T)/dt, where t is time, E is the exhaust gas flow, and T is the exhaust gas temperature upstream of the SCR catalyst and establishing a signal related to the value d(E*T)/dt; and calculation of the adjusted amount of reductant by means of the signals responsive to the required amount of reductant and of the value d(E*T)/dt.