## 2 Experimental Apparatus and Methodology  
The measurements were performed on an optically accessible single cylinder DI-SI engine. It was equipped with the four-valve cylinder head of a commercial SI power unit, a centrally located discharge spark plug, and wall-guided injector. The fuel system featured a commercial six-hole solenoid injector installed in the lateral side of the cylinder head (Figure 1). Further details on the engine specifications are reported in Table 1. Reference is made in crank angle degree (CAD) with respect to the top dead centre (TDC) at the end of compression, mentioned as after top dead centre (ATDC) and before top dead centre (BTDC). A conventional Bowditch design was used for optical accessibility [24], with an ultraviolet (UV) inclined (45\({}^{\circ}\)) mirror mounted within the hollow optical piston. The piston quartz window ensured a field of view of 57 mm in diameter, corresponding to 52% of the piston cross section. Slotted graphite piston rings were used to provide oil-less lubrication with uninterrupted bronze-Teflon rings used for sealing. Engine speed was fixed at 2000 rpm and the load was set at wide open throttle (WOT). Coolant and lubricant temperatures were maintained at 330-335 K using a thermal conditioning unit. Although under normal conditions of engine operation higher temperatures of the boundary conditions can be found, it is necessary to maintain a moderate temperature to protect the elongated piston-bore assembly that features high tolerance specific for optical engines. For comparative purposes between operating conditions, it will not have major effects. Intake air was at standard conditions (293 K and 1 atm) for all operative conditions. All measurements were performed with fixed spark advance (SA) of 15 CAD in order to maintain roughly the same fluid dynamic conditions (swirl, tumble, turbulence intensity, among others). In addition, this SA ensured a stable combustion process (COV\({}_{\text{IMEP}}\) \(<\) 3%) for all operative conditions. An experimental test matrix was designed with two injection strategies by varying the start of injection (SOI) of the second pulse (250 and 200 BTDC). Previous trials with more extensive SOI sweeps revealed that late injection tended to render the combustion process unstable, while early settings resulted in high smoke opacity [17]. Therefore, the investigation was focused on fuel delivery during the second half of the intake stroke and early stages of compression. For both cases (i.e., 250 and 200), three different split injection were used: double pulse with a proportion of 1:1 injected fuel (called double 50-50%), double pulse with a first injection higher than the second in a proportion of 2.3:1 (called double 70-30%) and triple pulse with the highest in the first pulse and equal for the other two in a proportion of 2:1:1 (called triple). In addition, single injection was also analysed for three different starts of injection settings. The baseline was taken as the case of single injection with the earliest SOI (300 BTDC), due to the representative condition of \"homogenous charge\" injection mode. More information of the injection strategies is shown in Table 2 and represented in Figure 2.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Case** & **First Injection** & \multicolumn{2}{c}{**Second Injection**} & \multicolumn{2}{c}{**Third Injection**} \\ \hline  & **SOI** & **injected** & **SOI** & **injected** & **SOI** & **injected** \\  & **[CAD** & **Proportion [\%]** & **[CAD** & **Proportion [\%]** & **[CAD** & **Proportion [\%]** \\  & **BTDC]** & **BTDC** & **BTDC** & **BTDC** & **BTDC** \\ Baseline & 300 & 100 & - & - & - & - \\ Single 250 & 250 & 100 & - & - & - & - \\ Single 200 & 200 & 100 & - & - & - & - \\ Double & 300 & 50 & 250 & 50 & - & - \\  
In-cylinder pressure was acquired with an accuracy of \(\pm\)1%, and averaged over 200 consecutive engine cycles; the optical measurements were carried out for 30 cycles of each set of 200 stable cycles, due to camera memory limitations.