## 3 Results and Discussions  
### Gaseous Emissions  
The exhaust gaseous emissions results are shown in Figure 3. The overall lambda (i.e., actual air/fuel ratio over the stoichiometric air/fuel ratio) was found to be similar for the tested fuels as 2.9, 2.84, and 2.82 for diesel, E15B35D50, and E15B35 FTD50, respectively. This is an indication that the difference between the fuels was the direct result of the fuel composition. The CO emissions with E15B35 FTD50 and E15B35D50 increased compared to the reference fuel for the test condition. This could be a reflection of incomplete combustion occurring during the combustion process. In-cylinder pressure and detailed analysis of E15B35 FTD50 and E15B35D50 fuel blends combustion process was reported in a previous work [50]. The lower combustion temperature due to the higher heat of vaporization of the ethanol is among the factors that might have influenced the higher CO results. Thus, the cooling effect of the alcohol dominates the effect of the total oxygen content of the blend of fuels, which might suppress the CO oxidation process [51]. Moreover, the lower cetane number of ethanol increases the time for ignition (longer ignition delay) of the air-fuel mixture [52]. Further, the higher viscosity of biodiesel has been indicated to promote poor atomization and hence increases CO emissions [53]. A recent study has reported that blends of F-T diesel and biodiesel resulted in slightly higher CO emission and the researchers attributed this to the insufficient evaporation and short mixing time of the blend during premixed combustion [13]. The application of a diesel oxidation catalyst could effectively reduce these CO levels from the combustion of these alternative fuels [50]. A slight decrease in CO\({}_{2}\) emissions was observed when the engine was fueled with E15B35D50, while CO\({}_{2}\) emissions for E15B35 FTD50 are slightly higher than in the case of diesel fuel. This is in line with the carbon content of the estimated chemical formula of the fuel blends, though it has to be noted that the differences are within the confidence interval of the results. CO\({}_{2}\) is a common product of the combustion of hydrocarbon fuels so the  
Figure 3: Exhaust gaseous emissions of diesel, E15B35D50, and E15B35 FTD50. lower carbon content of ethanol molecules leads to lower CO\({}_{2}\) formation, even on an energy basis (g\({}_{\text{CO}_{2}}\)/M)\({}_{\text{fuel}}\)). In addition, for combustion stoichiometry, previous studies reported that an increase in CO\({}_{2}\) emission with the addition of biofuels (ethanol and biodiesel) is attributed to the higher oxygen content of the oxygenated, which favors the combustion process and hence increases CO\({}_{2}\) formation [54; 55]. The combustion of E15B35D50 and E15B35 FTD50 have reduced the THC emissions (mostly comprised of heavy unburned hydrocarbons) when compared to diesel fuel (Figure 3). It has been reported in the literature that ethanol and biodiesel addition to diesel blends increase [56] or decrease [14] the hydrocarbons emission. The total and the heavy hydrocarbons presented in the THC have decreased between 18-33% for E15B35 FTD50 in comparison with diesel or E15B35D50. This might be associated with the higher cetane number and the absence of aromatics of F-T diesel, contributing to the reduction in THC. [PERSON] et al. [57] discussed that a higher cetane number and lower aromatic content are the main reasons for the occurrence of a shorter ignition delay time. Moreover, the virtually zero sulfur as well as the absence of aromatics, have been mentioned to result in a reduction in the engine-out exhaust emissions (i.e., reduce HC development and lowering PM) [58]. It has been previously reported in the literature that F-T diesel reduces hydrocarbons emissions [59] in comparison with diesel fuel. In this work, the addition of the oxygenated biofuels (ethanol and biodiesel) further reduced the unburned hydrocarbons emissions (Figure 3).