## 1 Introduction  
With progressively increasing strict emission rules due to collapsing atmosphere conditions, decreasing internal combustion engine tailpipe emissions have become a hotspot point for researchers. Since diesel combustion is naturally heterogeneous, diesel engines are considered the primary source of contamination. They release significant pollutants, such as carbon monoxide (CO), nitrogen oxide (NOx), unburned hydrocarbons (UHCs), and particulate matter (soot) [1, 2, 3, 4, 5]. Low-temperature combustion (LTC) is a modern combustion strategy that has emerged recently. It encompasses various techniques, including partially premixed compression ignition (PPCI), homogeneous charge compression ignition (HCCI), and reactivity-controlled compression ignition (RCCI). Employing LTC techniques in diesel engines can result in lower NOx and soot emissions while preserving higher efficiency when compared to traditional diesel engines. The HCCI technique relies on the auto-ignition of a fully premixed air-fuel mixture [6, 7, 8]. The PPCI method combines HCCI and traditional diesel engine combustion modes. Diesel fuel is injected near the top dead center, avoiding the cylinder wall. In PCCI mode, cooled exhaust gas recirculation (EGR) and a low compression ratio are employed to prolong the ignition delay (ID) and lower the combustion temperature, which helps reduce NOx and soot emissions [9; 10; 11]. The RCCI combustion strategy is an advanced method derived from the PCCI approach. In this technique, a high-reactivity fuel, such as diesel, is directly injected into the cylinder in one or more stages, and the low-reactivity fuel, such as methane (CH\({}_{4}\)), hydrogen (H\({}_{2}\)), or gasoline, is injected through the intake port. This method leverages the varying reactivities of the fuels to control the combustion timing and duration, offering more precise control over the combustion process than other methods. This technique employs the effect of varying reactivities of fuels to regulate the combustion phase and duration, and it gives more control over combustion than the other approaches [12; 13; 14; 15; 16; 17]. Methane is considered a promising low-reactivity fuel due to its distinct features, including a poor hydrogen-to-carbon ratio, less ringing intensity and high resistance to knock, higher heating value, less carbon footprint, less adiabatic flame temperature, and fewer tailpipe emissions, making it a clean fuel favorable for internal combustion engine compared with other fossil fuels. Moreover, it is available, has a lower price, has no challenges related to infrastructure, and has no evaporation loss [18]. Table 1 compares methane and n-Heptane fuels' physical and chemical properties [19]. [PERSON], [PERSON], and [PERSON] concluded in their study that when using a CR of 17, CA50 advanced before TDC. A TDC can be delayed by raising the amount of EGR, which results in rising UHC emissions. To maintain an acceptable amount of UHC emissions, a lower compression ratio of 14 can be used to retard the CA50 [20]. [PERSON] et al. revealed in their article that NOx and soot emissions were attained consistently with Euro-VI when the RCCI engine ran at 1200-1800 rpm and 2 to 9 bar BMEP in the absence of EGR. The exhaust temperatures were maintained at a low level for the suitable reduction in total UHC emissions by employing catalytic after treatment. Furthermore, the thermal efficiency of the RCCI engine was either similar to or exceeded that of a conventional diesel engine [21]. [PERSON] et al. reported that methane/diesel fueling results in a significant load enhancement weighed against gasoline/diesel fueling for RCCI engines. The highest IMEP of 17.3 bar was accomplished when the CA50 occurred at 4 degrees after the top dead center. The findings demonstrate that the load limit can be increased through the utilization of a dual-fuel methane/diesel RCCI engine [22]. [PERSON] et al. stated that natural gas's lowest methane number (MN) reduced overall UHC emissions and methane slip.