# Dual-Fuel Combustion for Future Clean and Efficient Compression Ignition Engines
## 1 Introduction
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 [PERSON] 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, [PERSON] 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 [PERSON] 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, [PERSON] 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 [PERSON]. 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 autoginition 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 [PERSON] 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 great 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.