## 3 Results and Discussion
### NO\({}_{x}\) Emissions
Under low-speed operating conditions, the volumetric efficiency was boosted, which increased the air intake and, consequently, the induction of O\({}_{2}\) within the cylinder and the formation of NO\({}_{\rm x}\). The engine speed had an impact on both the volumetric efficiency and the heat transfer from the cylinder. The volumetric efficiency dropped at higher engine speeds, leading to a higher degree of residual gas dilution in a manner similar to a miniaturization of the effect obtained from the dilution obtained from exhaust gas recirculation. Increasing the engine speed produced two conditions with contradictory effects on the heat transfer rate: A reduced cycle time, which diminished the heat transfer rate; and higher turbulence, which enhanced the heat exchange process. However, the former effect dominated, with the reduced heat transfer rate leading to higher burned gas temperatures at higher speeds. The relatively long residence time of the NO\({}_{\rm x}\)-forming reaction kinetics associated with lower speeds is another factor affecting NO\({}_{\rm x}\) formation. The net effect of these respective factors is the set of NO\({}_{\rm x}\)-formation trends shown in Figure 6, which are specific to the combusted fuel, engine specifications, and operating conditions. Overall, at higher engine speeds, NO\({}_{\rm x}\) formation is suppressed by a lack of O\({}_{2}\) and an inadequate residence time. These findings agree closely with extensively reported results in the literature [58; 59]. Following the acceleration of the engine from 1600 to 3600 rpm under full loading, a reduction of 38% in NO\({}_{\rm x}\) emissions was observed in both cases where the fossil-based diesel and B20 were used as fuel. This reduction increased to 42% under a half-loaded condition, suggesting that reducing the load made the engine more responsive in terms of NO\({}_{\rm x}\) formation to the speed variation. Under low loading and high-speed conditions, three restrainers (lower temperature, lack of O\({}_{2}\), and shorter residence time) act against NO\({}_{\rm x}\) formation. At higher loads and under any speed and fuel condition, the engine produced higher amounts of NO\({}_{\rm x}\) emissions. Increasing the load increased the amount of fuel burned within the combustion chamber, resulting in a higher temperature, which is the primary contributor to NO\({}_{\rm x}\) formation. This finding is also reflected in the literature [47]. Increasing the fraction of DSO biodiesel in the blend also promoted NO\({}_{\rm x}\) formation at all loads and speeds because biodiesel fuel has more inherent O\({}_{2}\) in its chemical composition. When adequate amounts of O\({}_{2}\) were available within the combustion chamber, the fuel charge underwent complete combustion, which in turn raised the temperature of the product gas. As noted above, a high amount of available oxygen at a high temperature corresponds to a convenient environment for NO\({}_{\rm x}\) formation via the Zeldovich mechanism. Accordingly, increasing the fuel-bound oxygen increased the NOx in the exhaust. The increased NO\({}_{\rm x}\) emission can be also partially attributed to the compressibility features of DSO biodiesel in terms of its bulk modulus, which quantifies how easily the volume of a quantity of fuel can be varied by applying pressure (alternatively, the bulk modulus governs the pressure increase resulting from a sudden decrease in fuel volume). Biodiesel has a smaller bulk modulus of compressibility than fossil diesel. Therefore, at a given injection pump timing, the DSO biodiesel exhibited a faster pressure increase and an earlier injection timing of approximately 1\({}^{\circ}\) of crank rotation [60]. This advancing of the injection timing increased the NOx emissions. Several studies have also attributed the increases in NO\({}_{\rm x}\) emissions to the elevated levels of CN in biodiesel [61; 62]. It is believed that higher CN levels shorten the ignition delay, thereby advancing the combustion and increasing NO\({}_{\rm x}\) emissions.