Meeting future NOX emission regulations by adding an electrically heated mixer

Abstract

New regulations by the California Air Resources Board (CARB) demand a stringent 0.02 g/hp-hr tailpipe NOx limit by the year 2027, requiring Selective Catalytic Reduction (SCR) catalysts to provide high NOx conversions even at low (below 200°C) exhaust temperatures. This work describes utilizing an Electrically Heated Mixer System (EHM system) upstream of a Light-Off Selective Catalytic Reduction (LO-SCR) catalyst followed by a conventional aftertreatment (AT) system containing DOC, DPF, and SCR, enabling high NOx conversions meeting CARB’s NOx emission target. The AT catalysts were hydrothermally aged to Full Useful Life. Conventional unheated Diesel Exhaust Fluid (DEF) was injected upstream of both the LO-SCR and primary downstream SCR. The EHM system allowed for DEF to be injected as low as 130°C upstream of the LO-SCR, whereas, in previous studies, unheated DEF was injected at 180°C or dosed at 130°C with heated DEF. The combination of unheated DEF, EHM system, LO-SCR, and downstream SCR enabled the needed increase in NOx efficiency in low exhaust temperatures, which was observed in drive cycles such as in cold-FTP, LLC, and World Harmonized Transient Cycle (WHTC). There were several-fold reductions in tailpipe NOx using this configuration compared to its baseline: 3.3-fold reduction in FTP, 22-fold in Low Load Cycle (LLC), 38-fold in Beverage Cycle, 8-fold in “Stay Hot” Cycle, and 10-fold in WHTC. Finally, it is shown that the EHM system can heat the exhaust gas, such as during a cold start, without needing additional heating hardware integrated into the system. These results were observed without performing changes in the engine base calibration.