Emission control system

A method for controlling delivery of exhaust stream additive to an exhaust aftertreatment system with more than one exhaust aftertreatment device coupled to an automotive engine, comprising the steps of:

• computing the quantity of NOx and reducing species constituents of engine feedgas;
• computing desired exhaust stream additive ratio, that is, the ratio between exhaust stream additive to NOx, for a first exhaust aftertreatment device to achieve high exhaust system conversion efficiency in said first exhaust aftertreatment device;
• computing exhaust stream additive quantity which must be injected from an exhaust stream additive supply into the exhaust stream entering said first exhaust aftertreatment device such that said desired exhaust stream additive ratio is attained;
• decreasing said quantity of exhaust stream additive to account for the reducing ability of feedgas constituents;
• computing the amount of NOx exiting said first exhaust aftertreatment device based on NOx entering said first exhaust aftertreatment device less the amount converted in said first exhaust aftertreatment device;
• computing the quantity of reducing species exhausted from said first exhaust aftertreatment device;
• computing said desired exhaust stream additive ratio for a second exhaust aftertreatment device to achieve high exhaust system conversion efficiency;
• computing the quantity of exhaust stream additive which must be injected from said exhaust stream additive supply into the exhaust stream entering said second exhaust aftertreatment device such that said desired exhaust stream additive ratio is attained;
• decreasing said quantity of exhaust stream additive to supply to said second exhaust aftertreatment device to account for the reducing ability of feedgas constituents to said second exhaust aftertreatment device;
• and commanding the exhaust stream additive injectors to supply said exhaust stream additive.