## 3 Results and Discussion
### CO Emissions  
Figure 4 shows the impacts of the DSO biodiesel fraction on the amount of CO emitted by the engine running at different speeds and loads. Carbon monoxide is a silent killer because it cannot be perceived by visual or olfactory means, and a CO fraction of 0.5% in the air can cause unconsciousness and kill within 15 min. Even extremely low concentrations of 0.04% can cause headaches and can kill following exposure for several hours. Internal combustion engines (ICEs) are the main anthropogenic source of CO gas and cause more than 95% of CO emissions in urban areas [43]. Carbon monoxide is generated during fuel-rich charge combustion as a result of an oxygen insufficiency in the combustion chamber that prevents full oxidization of the carbon in the fuel. The combustion reaction in a CI engine comprises two pathways--the breakdown of the fuel to form CO, and the oxidation of CO to form CO\({}_{2}\)--with most of the fuel's energy liberated in the second process. The factors that determine the CO emissions from a CI engine include the fuel-air mixture features (such as the air-fuel ratio (AFR), mixing condition, and fuel type), engine operating conditions (such as speed, load, fuel atomization, and injection parameters including timing, duration, and pressure), and engine specifications (such as compression ratio and combustion chamber design) [44; 45]. Of these, the AFR has the largest influence on the production of CO emissions and can be seen as the prime driver of CO production, while the other factors enhance its effects. Thus, a CI engine produces lower CO emissions than a spark ignition engine because it operates under leaner overall conditions (AFR \(>\) 1). Unlike NO\({}_{\rm x}\) formation, the formation of CO is firmly driven by the initial stages of the combustion reaction, during which the fuel distribution within the combustion chamber is inhomogeneous. That is, the AFR is nonuniform and varies both spatially and temporally within the cylinder, producing fuel-rich regions that are optimal for CO formation. A jet of injected fuel penetrates the combustion chamber, entraining hot gas that, in turn, heats up the fuel and causes evaporation. The hot fuel vapor-air mixture forms a sheath surrounding the jet, including the jet tip. Pre-flame combustion begins at this stage (at a temperature of approximately 480 \({}^{\circ}\)C), drawing more hot air into the fuel jet and increasing the temperature to approximately 550 \({}^{\circ}\)C [46]. As the temperature increases continuously, the reaction rate of the combustion increases and, eventually, premixed combustion in which CO forms rapidly in the flame reaction zone begins and then increases to a peak value. The diffusion flame surrounding the spray plume is also established during this stage. Ultimately, diffusion combustion occurs at a temperature of approximately 1,600 \({}^{\circ}\)C. If a sufficient amount of hot oxygen is available, the CO produced by this path of the combustion reaction is oxidized to CO\({}_{2}\). This flame plume structure and its respective zones [46] are illustrated in Figure 5.  
Figure 3: Cont. Figure 3: CO\({}_{2}\) 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. Figure 4: Cont. Figure 4: CO 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. It is seen from Figure 4 that, under all loads and for all of the fuel types, the CO emissions increase moderately when engine speed increases. This trend in terms of engine speed is consistent with the reported literature [47; 48]. When the engine is operated at higher speeds, less time is available for the intake process, resulting in reduced volumes of air and richer mixtures within the cylinder.