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Influence of exhaust gas temperature and air-fuel ratio on NOₓ aftertreatment performance of five large passenger cars

Abstract:

Exhaust aftertreatment systems are crucial to ensuring real-world NOₓ emission limits for motor vehicles. Operating conditions constrain the NOₓ reduction performance of aftertreatment devices. This study analysed real-world NOₓ emissions, tailpipe exhaust gas temperatures, and air-fuel ratios during cold start in a closed-loop urban route, followed by hot-start real driving emissions (RDE) tests. Five Euro-6b sport utility vehicles (SUV) were tested: two gasoline vehicles with three-way catalyst (TWC), namely, one gasoline direct injection (G-DI) and one hybrid electric vehicle (HEV); three diesel vehicles with different NOₓ control systems, namely, only exhaust gas recirculation (EGR), lean-burn NOₓ trap (LNT), and selective catalytic reduction (SCR). The only-EGR- and LNT-equipped diesel vehicles and the G-DI vehicle surpassed the NOₓ Euro 6 limits in all tested sections. For the same vehicles, the total RDE emission factors were 9.0, 7.4, and 5.0 times the Euro 6 limits, respectively. In contrast, the diesel vehicle with SCR had an RDE emission factor 1.0 times the limit, and the HEV exhibited very low emissions at approximately 2 mg NOₓ km−1. However, during the cold start phase (first 5 min), the emission levels of the SCR and HEV vehicles surpassed the Euro 6 limits by 2.7 and 1.1 times, respectively. Based on the measurements at the tailpipe, the results indicate that cold start, urban driving, and cooling conditions of aftertreatment devices can lead to a decrease in the NOₓ conversion efficiency of TWC and SCR systems. The air-fuel ratio was key for the NOₓ conversion in TWC aftertreatment. The large differences between G-DI and HEV vehicles were primarily attributed to the lean and rich operations of the G-DI and HEV engines, respectively. To comply with stringent future regulations, lean-burn engines would require diesel-like aftertreatment. SCR and hybrid vehicles would require a careful aftertreatment thermal management or heating to further exploit their potential for reducing emissions in urban areas.