## 1 Introduction
However, this betterment was combined with increased NOx emissions. Furthermore, the combination of RCCI combustion and the late-intake valve closing (LVC) strategy can lead to reductions of up to 80% in methane slip and nitrogen oxide (NOx) emissions while simultaneously achieving relatively higher net indicated efficiency compared to the conventional dual-fuel operation [23]. In their study, [PERSON] et al. suggested increasing the proportion of natural gas in the fuel mixture, advancing the start timings of both the first and second injections beyond a certain threshold, and enhancing the fuel fraction during the first injection phase to reduce soot and NOx emissions. However, this approach had a detrimental effect on UHC and CO emissions [24]. [PERSON] et al. disclosed that using natural gas with high methane numbers will reduce the combustion time, and the ignition delay will be extended in NG/diesel RCCI engine. Additionally, the impact of the methane number on UHC and CO emissions in RCCI mode remains uncertain. Furthermore, combustion phasing has a greater influence on NOx missions than the composition of natural gas in RCCI operation [25]. [PERSON] et al. performed an optimization study on a heavy-duty single-cylinder diesel engine, comparing the performance of methane/diesel and gasoline/diesel RCCI configurations. The multi-dimensional CFD code, KIVA3V, is coupled with the Nondominated Sorting Genetic Algorithm (NSGA-II) using the CHEMKIN chemistry tool to carry out the optimization study for an immense range of operational parameters. Engine operational parameters,including the double injection timing of diesel, diesel pressure, the methane mass, and the amount of the EGR fraction while the objective was to minimize NOx, UHC, CO, and soot, NOx, and CO emissions, together with ringing intensity and ISFC, are used to control by the NSGA-II. The study concluded that methane is a superior choice as a low-reactivity fuel compared to gasoline for RCCI mode, primarily due to the greater reactivity difference, which aids in controlling the peak pressure rise. However, despite this advantage, the efficiency of the gasoline/diesel configuration is 2% higher than the methane/diesel configuration, attributable to the higher flame temperature of methane, which results in greater energy losses; in contrast, the methane/diesel RCCI mode produced lower NOx emissions. Furthermore, methane in the RCCI engine promotes smooth, clean, and efficient combustion. A minimal amount of soot and NOx emissions was achieved up to a load of 13.5 bar IMEP without the need for EGR. However, at higher loads, the study found that both combustion characteristics and tailpipe emissions were highly sensitive to the diesel mass in both injection phases [26]. [PERSON] et al. conducted an optimization study on a light-duty, inline four-cylinder dual-fuel RCCI engine operating on diesel and natural gas, which works on a load range from 3 to 12 bar IMEP. A design experiment (DOE) (2\({}^{3}\) factorial design) was executed to discover the effect of input parameters to control the desired outputs. The objectives and constraints are limited to meeting light-duty tailpipe emissions standard Tier 3 Bin 20. This optimization is employed with and without the execution of the after-treatment system while decreasing operational costs by minimizing the usage of brake-specific fuel and brake-specific urea. The study concluded that the urea fuel cost for operating the RCCI engine at higher loads (IMEP \(>\) 6 bar) was found to be 30-35% lower than that of a conventional diesel engine. Additionally, the optimal exhaust gas temperature range for CDC-RCCI operation was identified as 400-425 \({}^{\circ}\)C, as this allows the diesel oxidation catalyst (DOC) and selective catalytic reduction (SCR) system to operate with high efficiency, ensuring compliance with EPA emission regulations.