## 1 Introduction  
Gasoline direct injection (GDI) together with downsizing and boosting is surely the most attractive answer to the demand for high energy efficiency of spark ignition (SI) engines [1]. In addition, the development of new SI engines is needed to fulfil the new EU6 emission standards [2]. This mean an important reduction of exhaust pollutants (HC, CO, NO,) with special attention in the last few years to particulate emission [3]. Direct injection (DI) has advantages over the port-fuel injected (PFI) engines, such as high precision in fuel metering [4], greater flexibility for different operation regimes, low pumping losses and significant potential in fuel economy benefits [5]. Another advantage is the greater charge cooling potential, which allows higher compression ratios to be used, increasing thermal efficiency [6]. In general, GDI engines are operated at stoichiometric combustion mode to ensure high efficiency of the three-way catalytic converter (TWC) [7]. Flexible control of the air-fuel ratio in the combustion chamber, typical of DI engines, is due to the possibility of realizing either stratified charge or homogeneous mixtures. This is possible by changing the injection pulse while maintaining the overall air-fuel ratio at the stoichiometric point [8], and features the potential for applying advanced operating regimes. [PERSON] et al. [9] reported that injection during the intake stroke (\"homogeneous mixture\") improves volumetric efficiency. Furthermore, injection during the latter stage of compression stroke (\"stratified charge\") suppresses the tendency towards knock. Several researchers studied stratified injection mode [10; 11; 12; 13; 14; 15] with significant improvements in performance. However, several problems were found in the form of a reduction in combustion stability, difficulties setting the desired air-fuel ratio at spark timing and an increase of particle emissions [13]. Therefore, a potential alternative is the mix of both concepts to achieve a stable combustion process, with low particle emission and without losing performance. Some authors have studied this strategy of homogeneous in-cylinder charge with single [16; 17], and multiple injections [18; 19; 20] in the intake and early compression stroke. [PERSON] et al. [18] found that the split injection strategy introduces positive effects with respect to the single injection case, such as the reduction of coefficient of variation of the indicate mean effective pressure (COVIMEF), tendency to knock, carbon monoxide emissions, as well as soot formation. However, many different injection permutations are possible, so there is no clear agreement on what constitutes an ideal strategy for each operative condition [21]. Therefore, more fundamental studies are necessary for better understanding of the first and late phase of the combustion process. Optical accessibility offers high temporal and spatial resolution, a significant advantage over the well-established in-cylinder pressure-based investigations. For example, the results of [PERSON] et al. [19] show that retarded second injections in the intake stroke lead to a less circular flame, potentially as a result of decreased charge homogeneity causing local regions of varied laminar flame speed throughout the combustion chamber. The spray dynamics and interaction with the surrounding air flow were recognized as strongly affecting mixture quality, combustion development and amount of pollutants released at the exhaust [22]. Spray impact against walls also needs to be studied, since undesired deposition of liquid as wall film causes unburned hydrocarbons and soot formation, hence increased fuel consumption and even augmented cycle-by-cycle variations [23]. It is therefore necessary to understand the mechanics behind injection strategies, which are capable of reducing particle emissions while also maintaining the performance and efficiency of the engine.