**1. Introduction**

Pollutant emissions regulations have become more and more stringent in recent years, especially for conventional diesel combustion (CDC) engines. Due to the existing trade-off between NOx and soot emissions in compression ignition (CI) engines, manufacturers had to incorporate diesel particulate filter (DPF) and selective catalyst reduction (SCR) aftertreatment systems to remove these two pollutants from the exhaust gas down to the limits imposed by the emissions regulations, such as EURO VI. The addition of these systems to the engine unit directly impacts to the production costs and also implies a higher level of complexity [1]. In addition, the efficient operation of these systems requires the consumption of some extra fluids in the exhaust line, such as urea upstream of the SCR to reduce NOx emissions and diesel fuel downstream the turbine to rise the exhaust gas temperature during the heating-up period of the engine. Moreover, the aftertreatment systems generate an extra back pressure, increasing also the in-cylinder fuel consumption.

Several combustion strategies have been investigated with the aim of reducing the costs caused by using aftertreatment systems [2]. These strategies are intended to keep the benefits of CDC operation in terms of performance and improve the engine efficiency [3]. To achieve this, many researchers have focused on the low temperature combustion (LTC) strategies [4]. As the literature demonstrates, the LTC strategies mitigate the NOx and soot formation by promoting a highly diluted in-cylinder fuel-air mixture [5] and extended premixing time between fuel and air prior to combustion [6,7], which breaks

the NOx-soot trade-off with CDC strategies. Moreover, the efficiency of the engine is improved due to the heat transfer reduction, among other factors [8].

Within the LTC strategies, homogeneous charge compression ignition (HCCI) was widely investigated at institutions worldwide [9]. HCCI relies on achieving a homogenous fuel-air mixture prior to combustion, which contributes to reducing the NOx and soot formation [10]. This promotes a fast heat release at the autoignition time, which results in higher thermal efficiency than CDC if the combustion event is well-phased on the engine cycle. However, despite that HCCI seems thermodynamically attractive, the combustion onset is entirely governed by chemical kinetics, i.e., in-cylinder pressure, temperature, equivalence ratio, and fuel properties. In addition, the fast heat release during HCCI combustion leads to high pressure gradients, which provoke mechanical stress on the engine as well as excessive combustion noise as load increases. For these reasons, HCCI was found to be limited to partial loads [11].

Another LTC strategy deeply studied is the partially premixed combustion (PPC) [12]. This strategy relies on using low reactivity fuels and more delayed injection timings to overcome the HCCI weaknesses in terms of combustion control and knocking at high loads [13]. Researchers have demonstrated that using gasoline instead of diesel allows improvement of control over the heat release rate, providing low levels of NOx and soot [14]. On the other hand, several PPC studies with different octane number fuels showed that the higher the research octane number (RON), the higher the unburning problems and dispersion cycle-to-cycle, being critical for gasolines with RON higher than 91 [15]. In this sense, several authors explored the possibility of using a spark plug to improve the combustion control and reduce the unburned products at low load [16], but the advantages of PPC in terms of NOx and soot emissions were lost [17]. Considering that diesel fuel ignites easier than gasoline, Park et al. [18] decided to explore the effects of fuel blends formed by diesel and gasolines. In that work it is stated that the addition of gasoline to the blend provides a reduction in the fuel density, kinetic viscosity, and surface tension, improving the atomization process. In addition, it provides also high ignition delays enhancing a more homogeneous blend formation. Thereby, the trade-off between NOx and soot is reduced. On the other hand, it was also found that carbon monoxide (CO) and unburned hydrocarbons (HC) emissions increased substantially.

In this sense, the study performed by Bessonette et al. [19] suggested that the optimum fuel for LTC strategies depends on the engine operating conditions. In particular, at low loads a highly reactive fuel in necessary, but at high loads a low reactivity fuel is needed. Based on this statement, Inagaki et al. [20] proposed a premixed dual-fuel strategy that used two fuels of different reactivity. In particular, gasoline-like fuels were injected by a port fuel injector (PFI) and diesel was direct injected (DI) into the combustion chamber as ignition trigger. This concept allows the in-cylinder mixing of both fuels with different ratios, which allows obtaining the desired reactivity. The combustion onset was managed by varying the reactivity of the fuel blend. As suggested by Bessonete et al., a clean and efficient operation was achieved by promoting a low cetane number in-cylinder ambient at high load and a high cetane number at low load.

More recently, Kokjohn et al. [21] continued developing this technique and proposed the reactivity controlled compression ignition (RCCI) concept [22], which follows the same injection method as the dual-fuel PCI concept proposed by Inagaki. In particular, port fuel Injection (PFI) is used to inject gasoline or other low reactivity fuel and direct injection (DI) is used for diesel fuel [23], so that it is possible to adjust the fuel reactivity for the requirements at the different engine loads [24]. The low reactivity fuel is injected generating a premixed blend of fuel, fresh air, and EGR. Then, diesel is injected in one or more injection pulses. Later, when the autoignition conditions at the combustion chamber are reached, the combustion starts and drives into the burning of the premixed charge at the regions with highest reactivity fuel and propagates down the reactivity gradient, as described by Hanson et al. [25].

Several authors have demonstrated that RCCI provides ultra-low NOx and soot emissions and the same time that improves the fuel consumption versus CDC [26]. This is achieved through the combination of high gasoline fractions and optimized injection timings for diesel fuel, which was proven to be crucial to attain a clean and efficient combustion [27]. A grea<sup>t</sup> part of the efficiency gain as compared to CDC was demonstrated to be caused by the lower heat transfer losses, which are explained due to lower in-cylinder temperature because of the highly diluted ambient [21]. However, despite the advantages of this concept, the research community still has to face several challenges such as the low combustion efficiency at low loads [28], which reduces the potential of this concept and also can compromise the efficiency of the diesel oxidation catalyst (DOC). Continuing the investigation in this line, the present study focuses on actions that could be taken to improve the RCCI concept. For this purpose, several studies have been carried to extract some guidelines for minimizing the energy losses with RCCI. In particular, three different paths are examined in this work: the engine settings combination as a method to improve the combustion efficiency at low load, a piston bowl geometry modification to diminish the heat transfer losses, and a low reactivity fuel type variation to improve combustion.
