Ammonia Slip Estimation Based on Ammonia Slip Catalyst Control-Oriented Modeling and On-Board Diagnostics NO_X Sensor Cross-Sensitivity Analysis

The incoming emission regulations for internal combustion engines are gradually introducing new pollutant species, which require greater complexity of the aftertreatment systems concerning layout, control, and diagnostics. This is the case of ammonia, which is injected into the exhaust gas through urea injections for NO_x abatement in selective catalytic reduction (SCR) systems and can also be generated in three-way catalysts. However, ammonia slip requires its oxidation on a dedicated ammonia slip catalyst (ASC). The set composed of the urea injection system, SCR, and ASC requires control tools to ensure high NO_x conversion efficiency and compliant ammonia slip under real driving conditions. These tasks are based on the use of NO_x sensors ZrO₂ pumping cell-based, which has the disadvantage of high cross-sensitivity to ammonia that can affect the measurement of NO_x and compromise the SCR-ASC control strategies. The present work proposes a methodology to predict ammonia and NO_x tailpipe emissions. For this purpose, a control-oriented ASC model was developed to use its ammonia slip prediction to determine the cross-sensitivity correction of the tailpipe NO_x sensor. The model is based on a simplified solution of the transport equations of the species involved in the ASC reaction mechanism. The model was calibrated using steady- and quasi-steady-state tests performed in a Euro 6c engine. Finally, the performance of the proposed methodology to predict NO_x and ammonia emissions was evaluated against experimental data corresponding to worldwide harmonized light vehicles test cycles (WLTC) applying different urea dosing strategies.