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 vehicles 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 and G-DI vehicles 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. In addition, the air-fuel ratio was key to 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.

中文翻译：

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尾气温度和空燃比对5款大型客车NOₓ后处理性能的影响

摘要 尾气后处理系统对于确保机动车的实际 NOₓ 排放限值至关重要。操作条件限制了后处理装置的 NOₓ 还原性能。本研究分析了闭环城市路线冷启动期间的真实 NOₓ 排放、尾气管废气温度和空燃比，然后进行了热启动实际驾驶排放 (RDE) 测试。测试了五辆欧 6b 车辆：两辆配备三元催化器 (TWC) 的汽油车，即一辆汽油直喷 (G-DI) 和一辆混合动力电动车 (HEV)；三种具有不同 NOₓ 控制系统的柴油车辆，即仅废气再循环 (EGR)、稀燃 NOₓ 捕集器 (LNT) 和选择性催化还原 (SCR)。唯一配备 EGR 和 LNT 的柴油和 G-DI 车辆在所有测试部分都超过了 NOₓ Euro 6 限制。对于相同的车辆，总 RDE 排放因子分别是欧 6 限值的 9.0、7.4 和 5.0 倍。相比之下，带有 SCR 的柴油车的 RDE 排放因子是限值的 1.0 倍，而 HEV 的排放量非常低，约为 2 mg NOₓ km-1。然而，在冷启动阶段（前 5 分钟），SCR 和 HEV 车辆的排放水平分别超过欧 6 限值的 2.7 倍和 1.1 倍。基于排气管处的测量结果表明，冷启动、城市行驶和后处理装置的冷却条件会导致 TWC 和 SCR 系统的 NOₓ 转化效率降低。此外，空燃比是 TWC 后处理中 NOₓ 转化的关键。G-DI 和 HEV 车辆之间的巨大差异主要归因于 G-DI 和 HEV 发动机的贫油和富油运行。为了遵守未来严格的法规，稀燃发动机需要类似柴油的后处理。SCR 和混合动力汽车需要仔细的后处理热管理或加热，以进一步发挥其在城市地区减少排放的潜力。