## 3 Results and Discussion
### Influence of 50% Heat Released Crank Angle  
Figure 10 presents the relationships between the centre of combustion and peak in-cylinder pressure (Figure 10a), combustion duration (Figure 10b), brake specific fuel consumption (Figure 10c) and soot emissions (Figure 10d).  
The reasons for variations of crank angle corresponding to 50% of heat release were already discussed in the previous section with reference to Figures 8 and 9. The trends reported in Figure 10 are therefore a further consequence of variables controlled in the different experimental campaigns.  
From Figure 10a, it is clear that the combustion development was less favourable to achieve a higher level of in-cylinder pressure when \(\theta_{50}\) moved far from the TDC. This was due to the increase of EGR rate in Sets 1 and 2 or to the decrease of rail pressure in sets 3 and 4. The same result was observed in Reference [38]. Similar trends were observed for the three operating conditions in the different testing modes, with high levels of determination coefficient. The only exception was related to HP trade-off in point 2000 \(\times\) 5, due to the wider range of EGR rate, leading to large variations of intake pressure and charge mass flow rate.  
For the same reasons, the shift of combustion centre along the expansion stroke led to the longer combustion duration shown in Figure 10b when controlling CDI (increasing it in Sets 1 and 2) or actuating lower levels of \(\mathrm{p_{rail}}\) in Sets 3 and 4.  
Figure 10c shows that retarding the combustion process within the expansion stroke led to an increase of bsfc. It is interesting to note that in each set presented in this graph, variations between the minimum and maximum levels are above the measurement uncertainties. In fact, the maximum uncertainty on measured levels of bsfc is around 1.5% [20], while observed changes in Figure 10c are above 2% (point No.1), about 1.6% (point No.2) and between 1.7-5.3% (point No.3). The corresponding variations of \(\theta_{50}\) ranged from 0.5 up to 1.5 CAD, that is, slight changes in the combustion centre position had a significant consequence on fuel consumption.  
Similar results are presented in Reference [38; 39] when considering diesel oil, even if referring to indicated quantities. In the first study, increases of three CAD of the combustion centre position (with reference to the level corresponding to the maximum indicated thermal efficiency) led to average variations of about 2% in efficiency and fuel consumption. The second investigation reported a first isfc increase of about 5% when moving \(\theta_{50}\) of one CAD along the expansion stroke and a second rise around 4% for a further retard of two CAD.  
From Figure 10d, it is apparent that the lowest levels of soot emissions are obtained when \(\theta_{50}\) is closer to TDC for each test set, because of the reduced or zero EGR rate or the maximum value of rail pressure. These conditions led to a shorter combustion duration and, for Sets 3 and 4, to an enhanced premixed phase of the combustion, which reduced bsS emissions [37].  
The shift of \(\theta_{50}\) along the expansion stroke is related to worst conditions inducing soot formation, with lower oxygen availability and a prevailing contribution of diffusive combustion, which is commonly related to this emission.  
Graph 10d includes a large number of sets showing the link within the combustion centre and bsS, which also presented wide variations of bsS emissions, mainly when controlling the EGR rate. As happened for the other parameters reported in Figure 10, in Reference [38] the same relationship between \(\theta_{50}\) and bsS was presented.