## 3 Results and Discussion
### HC Emissions
The fossil diesel and DSO biodiesel blends demonstrated similar HC emission reductions of nearly 55% when the engine speed increased from 1600 to 3600 rpm at full load. At 50% loads, these reductions increased to nearly 75%. The higher sensitivity of the variation in HC emissions to changes in engine speed under lower loads is likely attributable to the higher concentrations of HC emissions produced at lower loads, which led to a greater amount of change in emissions with engine speed. Figure 7: Cont. Figure 7: HC variations with engine speed and load, presented as trends (left) and the normalized value contours (right) for (a) 100%, (b) 75%, and (c) 50% loading. (d) Comparison of the results for all fuels at all loads at 2000 rpm. Blending the neat fossil diesel with fractions from DSO biodiesel appears to also have had a significant impact on the HC emissions. As shown in Figure 7, increasing the amount of DSO biodiesel in the blend reduced HC emissions to a degree proportional to the DSO biodiesel fraction [47]. These substantial changes in emissions can be attributed to the variations in the ignition delay occurring as a result of blending the fossil-based diesel with the DSO biodiesel. It is worth mentioning that, in this context, the ignition delay was both a chemical and physical process. That is, it was driven by both the heterogeneous and the complex physical mixing process in the CI engine and the chemical kinetics of the fuel. All blends demonstrated considerable declines in HC emissions relative to neat fossil diesel throughout the running process, possibly because the higher CN content of the DSO biodiesel shortened the ignition delay and therefore decreased the fuel fraction that could be diluted by the surrounding air prior to ignition. As a result, the pre-mixed combustion phase was minimized, and less fuel had was dispersed within the cylinder prior to combustion. This reduction in ignition delay also constrained the amount of fuel that could impinge on the cylinder walls, a potential source of HC emissions. The best performance in terms of the maximum decrease in HC emissions relative to the benchmark diesel were attained at the mid-range of engine speed, with peak reductions of 13.9, 27.2, 34, and 44.4% at loads of 75, 100, 100, and 100% for B5, B10, B15, and B20, respectively. Further increases in the engine speed diminished these reductions and caused the HC emissions of the DSO blends to converge with that of the neat diesel. At the mid-range of engine speeds, the oxygen bound within the biodiesel fuels improved the combustion process, and the DSO biodiesel blends could meet acceptable levels of fuel atomization and homogeneous charge distribution within the combustion chamber. At higher engine speeds, however, extremely brief time intervals were available for any combustion event, including fuel atomization, evaporation, and distribution within the cylinder. The higher viscosities of the DSO biodiesel blends worsened the situation, leading to poor combustion efficiency. Eventually, the elevated HC emission levels approached that of neat fossil diesel, with the relative reductions for B5, B10, B15, and B20 reducing to 3.5, 14.2, 21.4, and 32%, respectively, at 50% load and 3600 rpm.