## 1 Introduction  
The future requirements regarding engine-out emissions and greenhouse gas (GHG) pollutants have been encouraging both engine original equipment manufacturers (OEMs) and the scientific community to search for alternative solutions to meet the normative constraints. Advances have been made in several systems such as injection devices (higher pressure pumps and faster injectors), combustion chamber geometry, and after-treatment devices. In parallel, new combustion modes are being developed aiming to reduce some of the raw emissions at a normative level. Among these combustion modes, the so-called low-temperature combustion (LTC) is receiving particular attention as this strategy can directly reduce the needs regarding after-treatment devices. This has fundamental importance, as these systems make up a large part of the final cost of the vehicle. The most expensive after-treatment system for vehicles operating with conventional diesel combustion is the selective catalytic reduction (SCR), which is used to reduce the levels of NOx emissions at the tailpipe. The second most expensive after-treatment system is the diesel particulate filter (DPF), which is needed to retain and oxidize the soot emissions before emission to the atmosphere. Finally, with a minor cost, a diesel oxidation catalyst (DOC) is needed to reduce the hydrocarbons (HC) and carbon monoxide (CO) emissions [1]. The major claim of the LTC concepts is the possibility to avoid the NOx-soot trade-off that occurs with conventional diesel combustion (CDC) [2] while simultaneously providing a high thermal efficiency [3]. One of the earlier investigated strategies was the homogeneous charge compression ignition (HCCI) mode. This combustion mode was able to achieve reduced NOx and soot emissions with efficiency levels similar to the diesel combustion modes. Despite these advantages, this method presented several challenges regarding combustion controllability and transient operation. In addition, its operation was limited to a narrow range due to high pressure gradients (upper limit) and excessive combustion inefficiency (lower limit). Over the years, several concepts were described in the literature such as the partially premixed combustion (PPC) [4] and spark-assisted PPC [5; 6]. Nonetheless, similar issues as the one presented with HCCI combustion do not permit the attainment of a TRL (Technology Readiness Level) higher than 6 (an exception to this is the SKYACTIV engine produced by Mazda). A recent change in the previous scenario was realized by the LTC concept called Reactivity Controlled Compression Ignition (RCCI). This concept was first presented by [PERSON] et al. [7] naming it as dual-fuel premixed compression ignition (PCI) combustion. Interesting results were addressed in their research, where extremely low NOx and soot emissions together with an excellent control of the combustion onset were reported. The main hardware advance was the use of two injector systems enabling the modification of the fuel percentages. Following these findings, [PERSON] et al. [8] continued developing this concept and renamed it as RCCI [9]. As the RCCI relies on using port fuel injection (PFI) for the low reactivity fuel (LRF) and direct injection for the high reactivity fuel (HRF), additional fuel lines are needed to supply both fuels. The fuel injected by the PFI system can also enhance the volumetric efficiency by reducing the charge temperature as demonstrated in the literature [10; 11; 12]. From reviewing the literature, it can be stated that the most used HRF and LRF are diesel and gasoline, respectively [13; 14]. Nonetheless, the RCCI was tested with a wide variety of fuels such as ethanol [15], methanol [16], and some others [17; 18; 19; 20] resulting in similar results as those obtained with gasoline.