A urea injection controller for a motorized system includes a passive regeneration model configured and disposed to calculate an amount of NOx conversion resulting from an interaction between exhaust gases and soot entrained in a selective catalyst reduction filter (SCRF) device, a replenishment mode trigger module configured to set an ammonia replenishment request based on the passive regeneration model, and a replenishment control module configured to selectively activate a urea injector to discharge a particular amount of urea based on the regeneration model.