## 4 Results
### Thermodynamic Analysis  
In-cylinder pressure was measured for all conditions presented in Table 2, and the results are shown in Figure 5. The earlier injection position Figure 5-left was not strongly affected by the split injection. The baseline case, double and triple injections showed similar pressure traces. However, for the single injection case with SOI 250 CAD BTDC, the decrease in terms of peak in-cylinder pressure was significant (over 25%). These results suggest that retarding the main injection from 300 to 250 CAD BTDC results in a reduction of fuel-air homogeneity and with this, a slower combustion process. The effect was even more evident for the SOI 200 cases. This is directly seen in the MFB curve (Figure 6a). As the spark timing was fixed, the variations in terms of performance (IMEP) seen in Figure 7 can be directly related to the changes in injection patterns. Figure 4: Raw and binary image for the analysis of the late combustion phase. Similar behaviour was seen when retarding the second injection pulse (Figure 5-right). For the case that the injected quantity during the second event was low (double 70-30), the pressure trace was similar to the baseline case. However, the decrease in terms of pressure was important for the case of single injection, double 50-50 and for the triple injection case. In these three cases, retarding the fuel delivery event(s) decreased overall combustion speed (Figure 6b) and with that, resulted in lower performance values (Figure 7). An overall conclusion is that splitting the injection event decreases the effect of injection retarding, most likely due to the better homogeneity between fuel and air. When looking at the  
Figure 5: In-cylinder pressure for different split injection strategies at fixed SA (15 CAD BTDC) and engine speed (2000 rpm); second injection phasing set at 250 (**a**) and 200 (**b**) CAD BTDC. Figure 6: Mass fraction burned for different split injection strategies at fixed SA (15 CAD BTDC) and engine speed (2000 rpm); second injection phasing set at 250 (**a**) and 200 (**b**) CAD BTDC. engine output values, all split injection strategies produced similar results to the baseline case, with only retarded single injection showing a decrease in IMEP. From this evaluation as well, placing the second injection at 250 CAD BTDC was found to ensure better results. An important observation is that despite operating at WOT and 2000 rpm, the IMEP values are only around 7.2 bar (equivalent to an output power of 4.8 kW). This phenomenon is attributed to the fact that in units with optical accessibility the top land region and crevice volumes are larger than in commercial engines, as is the mass lost due to blow-by past the sealing rings. This reduces the compression ratio and lowers efficiency (5 to 10%) with respect to conventional engines. In addition, this effect prolongs the last phase of the combustion process (MFB between 90 CAD ATDC and 150 CAD ATDC), as observed in other work of the research group [31]. On the other hand, this type of test bed provides valuable information in terms of in-cylinder phenomena. Therefore, it has been used by many researchers and companies in recent years [1,32,33]. One other issue is that the optical window is flat, while the setup found in the commercial engine from which the cylinder head was taken featured a profile designed for wall guided stratification (i.e. one of the six fuel jets is deflected towards the spark plug by the piston shape for injection during the late compression stroke). Even if there are differences in fluid motion with respect to real-world applications, the fact that fuel delivery was performed during the intake stroke, as well as the fact that comparative analyses were intended, should reduce the influence of these alterations to a minimum. Given the relatively high conversion efficiency of oxidation catalysts that cut CO and HC emissions, the main elements that benefit significantly from direct reduction inside the combustion chamber are NOs and particulate matter (PM).