Multi-stage SCR control and diagnostic system

An exhaust gas after-treatment diagnostic system for an internal combustion engine, comprising:

• a first selective catalytic reduction device receiving an exhaust gas flow produced by said internal combustion engine;
• a first NOx concentration detection means positioned upstream from said first selective catalytic reduction device generating a first NOx concentration signal;
• a second NOx concentration detection means positioned downstream from said first selective catalytic reduction device generating a second NOx concentration signal;
• a first dosing apparatus for delivering a reductant into said first selective catalytic reduction device, a dosing controller adapted to adjust a first reductant delivery rate of said first dosing apparatus according to a first reductant dosing command to control a first normalized stoichiometric ratio, which is equal to a ratio of a molar amount of ammonia generated from said reductant delivered by said first dosing apparatus to a molar amount of NOx in said exhaust gas flow, upstream from said first selective catalytic reduction device;
• and
• a diagnostic controller configured to generate a first diagnostic cycle in which said dosing controller is configured to control said first normalized stoichiometric ratio lower than a first stoichiometric reaction ratio, which is a molar ratio of ammonia to NOx in a reaction in said first selective catalytic reduction device, and calculate a first reductant quality ratio, which is indicative of a ratio of a NOx concentration change in said exhaust gas flow across said first selective catalytic reduction device to a concentration of said reductant delivered by said first dosing apparatus in said exhaust gas flow upstream from said first selective catalytic reduction device, in response to said first NOx concentration signal, said second NOx concentration signal, and said first reductant dosing command.

An exhaust gas after-treatment control system for an internal combustion engine, comprising:

• a first selective catalytic reduction device receiving an exhaust gas flow produced by said internal combustion engine;
• a first NOx concentration detection means positioned upstream from said first selective catalytic reduction device generating a first NOx concentration signal;
• a second NOx concentration detection means positioned downstream from said first selective catalytic reduction device generating a second NOx concentration signal;
• a first dosing apparatus for delivering a reductant into said first selective catalytic reduction device, a dosing controller adapted to adjust a first reductant delivery rate of said first dosing apparatus according to a first reductant dosing command to control a first normalized stoichiometric ratio, which is equal to a ratio of a molar amount of ammonia generated from said reductant delivered by said first dosing apparatus to a molar amount of NOx in said exhaust gas flow, upstream from said first selective catalytic reduction device;
• and
• a feedback controller configured to generate a first detection cycle in which said dosing controller is configured to control said first normalized stoichiometric ratio lower than a first stoichiometric reaction ratio, which is a molar ratio of ammonia to NOx in a reaction in a first selective catalytic reduction device, and calculate a first reductant quality ratio, which is indicative of a ratio of a NOx concentration change in said exhaust gas flow across said first selective catalytic reduction device to a concentration of said reductant delivered by said first dosing apparatus in said exhaust gas flow upstream from said first selective catalytic reduction device, in response to said first NOx concentration signal, said second NOx concentration signal, and said first reductant dosing command, wherein said feedback controller is further configured to calculate said first reductant dosing command in response to said first reductant quality ratio.