压燃发动机如柴油机具有高热效率、低油耗的突出优势，但面临氮氧化物（NOx）和颗粒物（PM）排放高的挑战。选择性催化还原器（SCR）和颗粒捕集器（DPF）是降低NOx和PM排放的主要手段。为了满足现行重型柴油机国Ⅵ排放法规要求，SCR通常布置在柴油机氧化催化器（DOC）与DPF下游，使得冷启动NOx转化效率较低。紧凑耦合SCR（cc-SCR）是布置在排气管出口的小型SCR催化器，具有热容量小、温升快的特点。加装cc-SCR是提高NOx转化效率、满足未来排放法规的有效措施。与柴油机相比，汽油压燃（GCI）发动机具有燃油喷射压力低、PM排放低等突出优点。本文针对柴油机双级SCR后处理系统实现超低NOx排放这一核心问题，围绕cc-SCR工作特性、cc-SCR最优氨存储、双级SCR尿素喷射策略以及双级SCR后处理系统对GCI发动机的适应性等开展相关研究。 通过尿素喷雾流场三维建模仿真结合台架实验，研究了温度、空速、氨氮比及尿素喷雾特性对cc-SCR工作特性的影响。研究结果表明，与无空气辅助系统相比，空气辅助系统尿素结晶质量更低，尿素喷雾在cc-SCR入口管道中生成更多NH3，相同条件下采用空气辅助系统时cc-SCR对NOx的转化效率更高。 基于催化器小样试验与台架试验，构建了cc-SCR化学反应动力学模型，并建立了快速升温过程NH3泄漏计算模型，结合多目标遗传算法优化分析cc-SCR催化器最优氨存储。研究结果表明，温度与温度升高率都会显著影响cc-SCR催化器最优氨存储。当催化器温度较低时，温度升高率对cc-SCR催化器最优氨存储影响更加显著。基于计算结果构建了温度-温度升高率-最优氨存储图谱，提出了cc-SCR瞬态氨存储模型适用性判别准则。 基于cc-SCR与主SCR催化器最优氨存储，提出了双级SCR尿素喷射策略。针对美国联邦测试瞬态循环（FTP）排气温度变化剧烈的特点，提出了柴油机负荷快速降低时的补充尿素喷射策略及基于NH3泄漏的反馈控制策略，结合原机热管理优化标定，实现了FTP循环下NOx排放低于0.027 g/kW·h，碳烟排放低于0.0068 g/kW·h，满足了加州CARB 2027超低NOx排放法规限值。 基于双级SCR后处理系统，在GCI发动机上研究了FTP循环排放特性及后处理系统的适应性。研究结果表明，GCI发动机NOx排放与柴油机接近，PM与颗粒物数量（PN）排放比柴油机更低，但碳氢（HC）排放高于柴油机。增加DOC催化器贵金属量或调整贵金属组分能够针对GCI发动机取得更高的HC转化效率。

Compression ignition engines such as diesel engines have advantages in thermal efficiency and fuel consumption with lean burn mode, while nitrogen oxide (NOx) and particulate matter (PM) emission need to be reduced. Selective catalytic reduction (SCR) and diesel particulate filter (DPF) are effective to reduce NOx and PM emissions from heavy-duty engine, and it is indispensable to satisfy China Ⅵ emission legislation with diesel oxidation catalyst (DOC) and DPF located upstream SCR. Under such condition, NOx conversion efficiency during cold start is limited. Close coupled SCR (cc-SCR) is a smaller SCR catalyst located close to the exhaust, which has the characteristics of smaller heat capacity and faster temperature increasing rate especially during cold start. Therefore, it is feasible to improve NOx conversion efficiency and meet increasingly stringent emission legislations with cc-SCR and integrated DOC+DPF+SCR aftertreatment system. Compared with diesel engines, lower fuel injection pressure and PM emission are observed with gasoline compression ignition (GCI) engines. In this thesis, the effect of dual-SCR aftertreatment system to achieve ultra-low NOx emission is investigated, and several researches such as the characteristics of cc-SCR, the optimal ammonia storage of cc-SCR, the control strategy of dual-SCR aftertreatment system and the adaptability of dual-SCR aftertreatment system to GCI engines are carried out. The effect of temperature, space velocity, ammonia nitrogen ratio and urea spray characteristics on cc-SCR are investigated by numerical simulation and engine bench test. It can be concluded the air-assisted urea nozzle is more suitable to be arranged upstream cc-SCR compared to airless urea nozzle considering its higher urea decomposition rate， NH3 concentration and NOx conversion efficiency. Based on the experimental results of sample test and engine bench test, SCR chemical reaction kinetics model is established and calibrated. Furthermore, a calculation model on ammonia slip in the rapid heating process is proposed and calibrated. The optimal ammonia storage of cc-SCR catalyst is analyzed combined with multi-objective genetic algorithm. It can be concluded that optimal ammonia storage of cc-SCR catalyst is affected by both temperature and temperature increasing rate, and the effect of temperature increasing rate on the optimal ammonia storage of cc-SCR catalyst is more significant at lower temperature. Based on the calculation results, the temperature-temperature increasing rate-optimal ammonia storage diagram is constructed, and the criterion for determining the applicability of ammonia storage model in transient state is proposed. Based on the optimal ammonia storage of cc-SCR and SCR, urea injection strategy for dual-SCR aftertreatment system is proposed and validated. To reduce the impact of severe temperature decrease on NOx conversion efficiency in federal test procedure (FTP) transient test cycle, the supplemental urea injection strategy and the feedback control strategy based on ammonia slip are proposed and validated. Combined with engine thermal management optimization calibration, tailpipe NOx and soot emission under FTP cycle are less than 0.027 g/kW·h and 0.0068 g/kW·h respectively, which indicates that tailpipe emissions for heavy-duty engine satisfy the California ultra-low NOx emission limit. Based on the the dual-SCR aftertreatment system, the emission characteristics and the adaptability of the aftertreatment system are studied on a GCI engine in FTP transient test cycle. It can be concluded that tailpipe NOx emission of gasoline compression ignition engine is close to that of diesel engine, tailpipe PM and PN emissions are lower than that of diesel engine. However, HC emission is higher than that of diesel engine, which can be appropriately reduced by optimized oxidation catalyst, such as increasing noble metal content or adjusting noble metal components.