## 4 Conclusions  
Extensive research and development remain to be carried out on second-generation biodiesel to investigate the extent to which they can overcome the barriers that have prevented the wide-scale adoption of first-generation biodiesel. However, there remain serious technical and economic hurdles that must be overcome before second-generation biodiesel can be deployed. Of the primary areas that require further clarification, combustion in automotive engines and the resulting tailpipe emissions are among the most prominent. To address these issues, this study investigated the tailpipe emissions produced during the combustion of biodiesel blends derived from DSO and analyzed the mechanisms of their formation during the combustion reaction in terms of the relation between biodiesel properties and the emission components. To that end, DSO was extracted and converted to biodiesel by applying the parameters and conditions reported in the literature, and its specifications were subsequently adjusted to conform with various standards. As a result, four DSO blends (B5, B10, B15, and B20) were prepared and used for testing in a CI engine to evaluate the resulting tailpipe emissions (CO\({}_{2}\), CO, NO\({}_{\rm X}\), and HC) relative to a fossil diesel comparison baseline. The following conclusions can be reported:  
1. The inclusion of DSO biodiesel in the blends reduced CO\({}_{2}\) emission levels, with a greater degree of enhancement occurring at lower engine loads and higher engine speeds. At full loading, an average reduction of emissions from B20 of 9.6% was obtained.
2. The CO emissions were reduced significantly in the case of DSO biodiesel blends. At full loading, the average CO emission reductions of 3.4, 9.7, 18.6, and 19.2% were obtained using B5, B10, B15, and B20, respectively.
3. The presence of inherent O\({}_{2}\) within the DSO biodiesel resulted in greater concentrations of NO\({}_{\rm x}\) emissions at all loads and speeds, with the peak (worst) increments of 2.8, 3.3, 5.4, and 7.4% obtained for blends B5, B10, B15, and B20, respectively. All of these peaks occurred under 50%loading. At higher speeds, the increases diminished, and the emissions approached the levels produced by fossil diesel.
* There were remarkable decreases in HC emission levels that were observed for the DSO biodiesel. The peak reductions of 13.9, 27.2, 34, and 44.4% were obtained at 75, 100, 100, and 100% loading for B5, B10, B15, and B20, respectively.  
Conceptualization, M.K., C.G., and A.G.O.; methodology, M.K., K.R., C.G., and I.T.N.; formal analysis, M.K., and A.G.O.; investigation, M.K., K.R., and C.G.; data curation, K.R., C.G., and I.T.N.; writing--original draft preparation, M.K., K.R., and I.T.N.; writing--review and editing, M.K., C.G., and A.G.O.; supervision, A.G.O.; funding acquisition, C.G.  
This research was funded by the University of Sharjah, UAE [grant number V.C.R.G./R.1285/2017].  
The authors would like to express their gratitude to the following institutions, laboratories, and inspection bodies for their support of this project: University of Sharjah, Lootah Biofuels LLC (UAE), Bureau Veritas-Inspectorate (UAE), Materials Laboratory (UAE), GHP Testing & Calibration Laboratory (UAE), and Dubai Central Laboratory (Food and Environment Laboratory Section).  
The authors declare no conflicts of interest.