Challenges and Opportunities for Application of Reactivity-Controlled Compression Ignition Combustion in Commercially Viable Transport Engines

Several advanced low-temperature combustion (LTC) strategies have been developed to reduce the harmful emissions from diesel engines. These LTC strategies, such as homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI), and reactivity-controlled compression ignition (RCCI), can reduce engine-out nitrogen oxides (NOx) and soot emissions simultaneously. LTC investigations exhibit several limi-tations of HCCI and PCCI combustion modes, such as lack of combustion control and other operational issues at higher engine loads, making their application in production-grade engines challenging. RCCI combustion mode exhibited promising results in combustion control, engine performance, and applicability at higher engine loads. The potential of the RCCI concept was demonstrated on different engine platforms, showing engine-out NOx levels below the limits proposed by the emissions regulations, together with ultra-low soot emissions, eliminating the need of after-treatment devices. However, the RCCI combustion mode has several challenges, such as excessive hydrocarbons (HC) and carbon monoxide (CO) emissions at low loads and excessive maximum pressure rise rate (MPRR) at high loads, which limit its effective operating range and practical applications. This review article includes recent advancements in RCCI combustion mode, its potential for using alternative fuels, the effects of different parameters on RCCI combustion mode and its optimization, and the ability of RCCI com-bustion mode to extend the engine operating limit to reach higher loads, which prevents the application of this concept in commercial applications. The findings of different optical diagnostics have also been included, which have been performed to understand the detailed chemical kinetics of the fuel-air mixtures and the effect of fuel reactivities on the RCCI combustion mode. The first part of this article focuses on these studies, which provide important outcomes that can be used for the practical implementation of RCCI combustion mode in production -grade engines. The second part of this article covers different RCCI combustion mode strategies that can be used to eliminate the restrictions of RCCI combustion mode at high loads. Among the different techniques, dual-mode concepts have been extensively investigated. The dual-mode concept is based on switching between two different combustion modes, typically an LTC mode and conventional compression ignition (CI) combustion mode, to cover the entire operational range of the engine. Many studies showed that the NOx and soot emissions from stationary engines with dual-mode RCCI/CI combustion had substantially improved versus a single-fueled CI combustion mode engine. Results related to the measurements of emissions and performance in transient con-ditions and driving cycles have also been included, which exhibit promising results for RCCI combustion mode. A comprehensive review on overcoming the challenges and real-world applicability of RCCI combustion mode is not available in the open literature yet. This article includes the results of relevant RCCI combustion mode in-vestigations carried out in single-cylinder and multi-cylinder engines, intending to fill this research gap. Finally, the results from alternative RCCI combustion mode concepts such as the dual-mode, hybrid-RCCI, simulations, and experiments in transient conditions using various driving cycles make this article uniquely relevant for researchers.