## 3 Results and Discussion
### NO\({}_{x}\) Emissions  
The nitrogen oxide (NO\({}_{\mathrm{x}}\)) emissions pattern produced by DSO biodiesel blends and fossil-based diesel are shown in Figure 6. As in the CO\({}_{2}\) and CO cases, the tests were conducted at different engine speeds and loads. Nitric monoxide (NO) and nitric dioxide (NO\({}_{2}\)) are collectively referred to as NO\({}_{\mathrm{x}}\), with NO representing the preponderant oxide (in diesel engine emissions, the proportion of NO\({}_{2}\) within NO\({}_{\mathrm{x}}\) is 10-30%) [54]. As very small concentrations of NO\({}_{\mathrm{x}}\) are emitted from a diesel engine relative to the aggregate exhaust emissions, the concentration of NO\({}_{\mathrm{x}}\) is expressed as parts per million (ppm). On-board ICEs contribute more than 50% of the NO\({}_{\mathrm{x}}\) that is emitted globally [55]. To better understand NO\({}_{\mathrm{x}}\) emissions trends with speed, load, and DSO biodiesel fraction, this study first discusses the formation of NO\({}_{\mathrm{x}}\) in a diesel engine. The air (composed of 21% O\({}_{2}\) as oxidant and 79% N\({}_{2}\) as diluent) is trapped inside the cylinder during the intake process and undergoes compression thereafter. The fuel is injected into this compressed and heated air at a few crank degrees before the top dead center (just prior to the onset of combustion), and the subsequent combustion reaction releases heat. If the temperature inside the cylinder does not exceed 1600 \({}^{\circ}\)C, the atmospheric (molecular) N\({}_{2}\) remains inert and is driven from the cylinder to the exhaust. Otherwise, the oxygen and nitrogen undergo dissociation and subsequent NO\({}_{\mathrm{x}}\) formation. In diesel engines, the gas temperature exceeds 1600 \({}^{\circ}\)C, making them the primary contributor to NO\({}_{\mathrm{x}}\) pollution. Although increasing the temperature within the cylinder increases the rate of NO\({}_{\mathrm{x}}\) formation, the formation reaction of NO\({}_{\mathrm{x}}\) is not itself part of the combustion reaction. Instead, the combustion creates the appropriate conditions and environment for NO\({}_{\mathrm{x}}\) formation. Three chemical mechanisms contribute to NO formation: A thermal (Zeldovich) process; the combustion of fuel N\({}_{2}\); and a prompt (Fenimore) mechanism [56]. As diesel and biodiesel fuels contain very small quantities of nitrogen of typically less than 2% by weight, the contribution of the fuel-bound N\({}_{2}\) is secondary, and the NO\({}_{\mathrm{x}}\) is not formed by the combustion of fuel. Prompt NO forms rapidly in the combustion flame reaction zone as a result of the reactions between intermediate chemical species. In CI engines, however, the flame front is extremely thin as a result the high pressure under which combustion occurs, and the residence time of the chemical species in the flame front zone is very short. As a result, NO formation via the thermal mechanism in the hot gases behind the flame front represents the predominant formation mechanism in such engines [57]. A typical stabilized combustion jet [46] of the type shown in Figure 5 is free from impingement or interaction with the combustion chamber walls and has a volume of hot combusted gas that is much greater than the instantaneous volume of the combustion flame front. Figure 6: Cont. Figure 6: NOx 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. To sum up the preceding discussion, NO\({}_{\rm x}\) formation in CI engines is governed by three main parameters: The temperature to which the burned gases can rise; the residence times of the chemical species; and the availability of oxygen. From Figure 6, it is seen that for all the fuels and under all loading conditions, the engine expelled lower amounts of NO\({}_{\rm x}\) emissions when running at higher speeds.