Apparatus, system, and method for reducing NOx emissions on an SCR catalyst

An apparatus for reducing NOx emissions in an engine exhaust gas stream of an engine system, comprising:

• a NOx reduction target module configured to determine a NOx reduction requirement, the NOx reduction requirement comprising an amount of NOx in the exhaust gas stream to be reduced on a selective catalytic reduction (SCR) catalyst;
• an ammonia target module configured to determine an ammonia addition requirement, the ammonia addition requirement comprising an amount of ammonia added to the exhaust gas stream to achieve the NOx reduction requirement, wherein the ammonia target module comprises a feedforward ammonia target module configured to determine a first ammonia addition requirement and a feedback ammonia target module configured to determine a second ammonia addition requirement, wherein the ammonia addition requirement is based on at least one of the first and second ammonia addition requirements, and wherein in a first operating mode associated with ammonia slipping from a tailpipe of the engine system the ammonia addition requirement is equal to the first ammonia addition requirement, and in a second operating mode associated with no ammonia slipping from the tailpipe the ammonia addition requirement is equal to the second ammonia addition requirement;
• a reductant target module configured to determine a reductant injection requirement, the reductant injection requirement comprising an amount of reductant added to the exhaust gas stream to achieve the ammonia addition requirement;
• and
• a reductant limiting module configured to determine a reductant limiting requirement representing whether at least one reductant limiting condition has been met;
• wherein the reductant target module is communicable in data receiving communication with the reductant limiting module to receive the reductant limiting requirement, the reductant target module being configured to limit the amount of reductant added in response to the reductant limiting requirement;
• and
• wherein the NOx reduction target module, ammonia target module, reductant target module, reductant limiting module, feedforward ammonia target module, and feedback ammonia target module each comprises one or more of logic hardware and executable code, the executable code stored on one or more non-transitory machine-readable storage media.