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