Model-Based Comparison of Passive SCR Aftertreatment Systems for Electrified Diesel Applications

The Diesel powertrain remains an important CO2 reduction technology in specific market segments due to its inherent thermodynamic combustion efficiency advantages. Diesel powertrain electrification can bring further potential for CO2 emissions reduction. However, the associated reduction in the exhaust gas temperature may negatively impact the performance of the exhaust aftertreatment (EAT) system and challenge the abatement of other emissions, especially NOx. Considering that active urea-SCR systems may be required to ensure compliance with the legislative limits, the total cost of the hybrid Diesel powertrain is expected to increase even more, therefore making it less commercially attractive. We present a model-based analysis of a Diesel hybrid electric vehicle (HEV) which is combined with an EAT system using Lean-NOx trap (LNT) technology. The overall de-NOx performance is further enhanced with the addition of passive selective catalytic reduction (SCR) to benefit from the on-board ammonia formation during rich combustion events. Since the modeling framework is fully physico-chemically informed, it allows the investigation of various topologies, catalyst geometrical and chemical properties. Moreover, the model includes a simplified virtual engine model and a virtual ECU to allow closed loop simulation of the engine + control + aftertreatment for various LNT/SCR control strategies. Four different powertrain topologies of the HEV system are considered to identify the optimal balance between the fuel consumption improvement and the negative impact on the EAT system. For the latter, seven different multi-brick EAT layouts are simulated, with different combinations of LNT, DPF and passive SCR/SCRoF components. Simulation results reveal that appropriate selection of the powertrain and EAT configuration, together with proper sizing of the components, can result in a cost-effective EAT system able to meet lower than Tier 3 Bin 125 emission limits.