## 3 Results and Discussion
### CO\({}_{2}\) Emissions  
Technically, the presence of CO\({}_{2}\) in the engine exhaust is a healthy sign of engine operation. Apart from CO\({}_{2}\), none of the principal diesel engine emissions shown in Figure 2 have adverse climate impacts. Even though it is not itself toxic, CO\({}_{2}\) is a prime greenhouse gas and the major contributor to global warming. Approximately 40% of the CO\({}_{2}\) produced by transportation, power generation, and industry is eliminated from the atmosphere by the plant photosynthesis process and through dissolution into surface water, which contributes to ocean acidification. The 60% of CO\({}_{2}\) emissions that remain in the atmosphere contribute significantly to climate change. Figure 3 shows the variations in CO\({}_{2}\) emissions by the respective fuels at different speeds and loads. In all of the blends and the benchmark diesel fuel, CO\({}_{2}\) emissions decrease as the engine load decreases or the engine speed increases. Running the engine under a partial loading reduces the amount of fuel delivered to the cylinder and, ultimately, the CO\({}_{2}\) produced by the combustion. This interpretation ties into the clear and worthy goal of reducing engine CO\({}_{2}\) emissions through the efficient use of combustion energy, i.e., converting the combustion energy into kinetic energy without waste. To achieve this, the losses in the various engine accessories can be minimized, kinetic and heat energy losses can be recovered, and the air resistance and drag from the motion of the car can be reduced. The average CO\({}_{2}\) emissions of diesel fuel over the entire speed range was reduced from 8.76 vol.% under full loading to 6.9 and then 4.6 vol.% under 75 and 50% loading, respectively. The average CO\({}_{2}\) emissions of B20 at 100, 75, and 50% loading were 7.9, 6.4, and 4.2% vol., respectively. When the engine ran at higher speeds, very brief time intervals were available for the intake process, resulting in a reduced intake of fresh air, which in turn diminished volumetric efficiency and, therefore, the CO\({}_{2}\) emissions in the engine exhaust [39; 40]. The presence of inherent oxygen in the DSO biodiesel offset some of this oxygen shortage within the combustion chamber. Thus, a reduction of 37% in the CO\({}_{2}\) emission of B15 at full loading was observed, compared to the 34.1% observed for the base diesel fuel, when the engine speed was increased from 1600 to 3600 rpm. For the same increase in engine speed and 50% engine loading, the corresponding decreases were 47.4 and 44.7%, respectively. It appears that, at lower engine loads, the variation in CO\({}_{2}\) emission became more sensitive to the increase in engine speed, which is the result of a combination of two effects that reduced both the carbon and oxygen levels in the cylinder. The inclusion of DSO biodiesel in the blends reduced the CO\({}_{2}\) emissions in proportion to the biodiesel fraction: At full loading, the average reductions in CO\({}_{2}\) emissions over the engine speed range were 2, 3.4, 5.7, and 9.6% for B5, B10, B15, and B20, respectively. This variation can be better understood by taking a closer look at the combustion chemistries of the respective DSO biodiesel (C\({}_{16}\)H\({}_{36}\)O\({}_{2}\)) species. The general stoichiometric combustion reaction can be expressed as:  
\[\ce{C_{n}H_{m}O_{w}+(n+m/4-w/2)O_{2}\to nCO_{2}+(m/2)H_{2}O}, \tag{1}\]  
Figure 2: Volumetric composition of typical diesel engine exhaust gas [38]. from which it is seen that the amount of CO\({}_{2}\) produced by the combustion is governed by the number of moles of each reactant. In the case of fossil diesel (C\({}_{12}\)H\({}_{23}\)), burning (n) moles produces (12n) moles of CO\({}_{2}\). By comparison, the combustion of (n) moles of the larger DSO biodiesel molecules produces (16n) moles of CO\({}_{2}\). However, it is more practical to analyze the combustion reaction in terms of mass. Fossil-based diesel has a smaller molar mass (167 g/mole) than DSO biodiesel (260.45 g/mole).