Selective Catalytic Reduction Control with Multiple Injectors

Over the past decade urea-based selective catalytic reduction (SCR) has become a leading aftertreatment solution to meet increasingly stringent Nitrogen oxide (NO_x) emissions requirements in diesel powertrains. A common trend seen in modern SCR systems is the use of "split-brick" configurations where two SCR catalysts are placed in thermally distinct regions of the aftertreatment. One catalyst is close-coupled to the engine for fast light-off and another catalyst is positioned under-floor to improve performance at high space velocities. Typically, a single injector is located upstream of the first catalyst to provide the reductant necessary for efficient NO_x reduction. This paper explores the potential benefit, in terms of improved NO_x reduction, control of NH₃ slip or reduced reductant consumption, of having independently actuated injectors in front of each catalyst. A dynamic programming approach is used to find optimal dosing strategies for both single and multiple injector installations over standard drive cycles. The resulting NO_x and NH₃ emissions for each case are compared in simulation to provide insight into the conditions where multiple injectors provide the greatest benefit. With these ideal settings, a second injector is found to be advantageous in situations where the control authority of a single injector is restricted but provide limited benefit elsewhere.