The global transition towards sustainable automotive vehicles has created a widespread demand for energy-efficient internal combustion engines with lower emissions (Ying Huang et al. 2023). Specifically, advanced aftertreatment systems, combined with in-cylinder innovations such as low-temperature combustion (LTC), can dramatically reduce particulate matter and nitrogen oxide (NOx) emissions, which are byproducts of the combustion process (Douadi O et al. 2022; Seongsu Kim et al. 2023). The development and optimization of these technologies are crucial for meeting increasingly stringent emission regulations while maintaining high engine performance and efficiency (André Chun et al. 2023; Tamilvanan A et al. 2023; Hamed Kazemi et al. 2019). This comprehensive review aims to provide an in-depth overview of the latest research advancements in aftertreatment methodologies, including selective catalytic reduction (SCR), lean NOx traps (LNT), and diesel particulate filters (DPF) (Gao J et al. 2019). The primary objective is to explore novel approaches for energy conversion and recovery that can enhance emission reduction capabilities and overall system efficiency. By examining the synergistic effects of pre-combustion and post-combustion purification technologies, this review seeks to identify strategies for effectively mitigating emissions while optimizing engine performance (Boretti A, 2020; Leach F et al. 2020).
Low-temperature combustion strategies, such as reactivity-controlled compression ignition (RCCI) and partially premixed compression ignition (PPCI), have emerged as promising techniques for overcoming the traditional NOx/soot trade-off inherent in diesel combustion (Venugopal I P et al, 2021; Suraj C et al. 2022). These advanced combustion modes operate at lower temperatures, avoiding the formation of both NOx and soot, regardless of the local equivalence ratio (Jiang Z et al. 2020). LTC has the potential to significantly reduce engine-out emissions, thereby relaxing the demands on aftertreatment systems. However, the implementation of LTC poses unique challenges for aftertreatment systems, particularly SCR, in low-temperature and cold-start conditions (Marzouk Osama, 2023; Zhang Y et al. 2021). SCR catalysts, which rely on the injection of a reducing agent (typically ammonia derived from urea) to convert NOx into nitrogen and water, face limitations in terms of catalytic activity and ammonia slip at low exhaust temperatures (Huang J et al. 2023). This issue is especially pronounced during cold starts when a significant portion of NOx is emitted. The light-off temperature required for efficient NOx reduction is often too high for real engine operating conditions, leading to unabated NOx emissions (Zheng J et al. 2023). Additionally, side reactions can lead to the formation of nitrous oxide (N2O), a potent greenhouse gas with a global warming potential 298 times higher than carbon dioxide. These challenges necessitate the development of advanced SCR catalysts with improved low-temperature activity, sulfur tolerance, and thermal stability (Wardana MKA et al. 2023). Lean NOx traps, which store NOx under lean conditions and reduce it to nitrogen under rich conditions, also face challenges in terms of storage capacity, regeneration efficiency, and durability. The integration of LNT and SCR systems has shown promise in enhancing NOx reduction performance, but further research is needed to optimize these hybrid configurations for LTC applications (Senthil R et al. 2023).
Diesel particulate filters, designed to capture and oxidize soot particles, must contend with the altered particulate matter characteristics resulting from LTC (Sun C et al. 2020). The lower exhaust temperatures associated with LTC can hinder passive regeneration, necessitating the development of advanced regeneration strategies and catalytic coatings to maintain DPF efficiency and durability (Wong SF et al. 2023) This review critically examines the current state of aftertreatment technologies and their integration with LTC strategies. It explores novel catalyst formulations, such as zeolite-based materials, perovskites, and mixed metal oxides, which have shown promise in enhancing low-temperature performance and durability. The review also investigates the potential of energy conversion and recovery techniques, such as thermoelectric generators and organic Rankine cycles, to harness waste heat from the exhaust and improve the overall efficiency of the aftertreatment system. Furthermore, this review delves into the complex interactions between engine operating parameters, combustion kinetics, and emission formation in LTC engines. By understanding the trade-offs between combustion efficiency, engine performance, and emissions, researchers can develop targeted strategies for optimizing both in-cylinder and aftertreatment processes. Advanced modeling techniques, such as computational thermal studies and kinetic simulations, are mediated as powerful tools for guiding the design and optimization of integrated LTC-aftertreatment systems. The review also highlights the importance of considering real-world driving conditions and transient operation in the development and evaluation of aftertreatment systems for LTC engines. Figure 1. shows a Schematic diagram of a typical LTC diesel engine setup. Cold starts, low-load operation, and frequent transients pose significant challenges for emission control, requiring adaptive and robust control strategies. The integration of advanced sensors, diagnostics, and control algorithms is explored as a means to ensure optimal performance and compliance with emission regulations under diverse operating conditions.
By providing a comprehensive analysis of the advancements and challenges in aftertreatment systems for LTC engines, this review aims to contribute to the development of sustainable and efficient automotive technologies. The insights gained from this study can guide future research efforts towards overcoming the limitations of current aftertreatment technologies in low-temperature conditions and achieving superior emission reduction performance in advanced combustion engines (Bakhchin D et al. 2023; Manjunath S P et al. 2023). The comprehensive nature of this review, covering the latest advancements in aftertreatment technologies, their integration with LTC strategies, and the consideration of real-world driving conditions, makes it a valuable resource for researchers, engineers, and policymakers working towards the development of clean and efficient automotive technologies. By providing a critical analysis of the state-of-the-art and identifying promising research avenues, this review aims to accelerate the progress towards sustainable transportation solutions that meet the growing demand for energy-efficient and environmentally friendly vehicles.