## 3 Results and Discussion
### HC Emissions  
Figure 7 shows the HC emission concentration levels in the exhaust gas produced at different engine speeds and loads with the respective DSO biodiesel blends. The concentrations shown in the figure are expressed in terms of the total hydrocarbon content and given in ppm. Hydrocarbon emissions (or organic emissions) in the exhaust of the CI reflect the hydrocarbons that escape the combustion process unburned. The composition of HC in CI engine exhaust is quite complex for two reasons: The diesel fuel itself comprises a combination of hydrocarbons with high molecular weight; and the structure of some hydrocarbons alters during the combustion reaction and does not complete the reaction, leading to the production of hydrocarbons that are not present in the parent fuel. Accordingly, unburned hydrocarbons comprise nearly 400 major and minor species. A normal combustion process in a CI engine involves four stages: Ignition delay, rapid combustion, mixing-controlled combustion, and late combustion. Fuel can escape this sequence unburned via four routes: (1) The over-mixing of air and fuel that causes the AFR to exceed the lean flammability limit; (2) under-mixing that makes the charge too rich to burn; (3) over-penetration of fuel spray that causes fuel droplets to impinge on the cylinder walls; and (4) sudden quenching of the flame during expansion or as a result of mixing with cold air. The most significant contributors to HC emissions from CI engines are over-mixing (over-leaning) and under-mixing (over-riching). It is seen from Figure 7 that increasing the engine load reduced HC emissions up to a specific load limit (75% load), with the trend reversing thereafter. This trend was also found by [42; 67; 68]. Increasing the engine load resulted in a richer mixture (lower AFR) within the combustion chamber and, therefore, a shorter ignition delay, corresponding to a reduced likelihood that the air-fuel mixture would be diluted by more air prior to autoignition. This made it unlikely that local mixtures that were lean to the levels below the flammability limit would develop. Ignition delay is strongly dependent on the air-fuel mixing condition and tends to be minimum near the stoichiometric proportion. However, a further increase in engine loading (to full loading) can prolong the delay period relative to the minimum condition. This in turn provides adequate time for the fuel to become further mixed beyond the lean limit of combustion, and therefore to avoid burning and the sustenance of a proper fast flame front. As a result, some of the fuel escapes through the exhaust port without being ignited. Under this interpretation, it is evident that ignition delay was the predominant factor governing the quantity of HC emissions in the assessments. At 3000 rpm, HC emissions of fossil diesel and B20 were 16.3 and 11 ppm, respectively, under the half-loading condition. These concentrations were reduced to 13.8 and 9.3 ppm, respectively, under 75% loading but increased again to 16.4 and 11.9 ppm, respectively, under full loading. For all of the fuels under all loading conditions, the concentration of HC emissions decreased as the engine speed increased. At higher speeds, the ignition delay was shortened, blocking the fuel from reaching the lean flammability limit, while the volumetric efficiency declined as a result of a lack of time available for the intake process. As a result, a rich mixture was produced within the cylinder, leading to complete combustion. At lower speeds, a sufficient amount of air was inducted within the cylinder, leading to a protracted ignition delay. Both effects promoted slow and incomplete combustion reactions that presented a favorable environment for unburned hydrocarbons, hydrocarbon fractionation products, and incomplete oxidation compounds. Some of these products avoided the combustion process and were ejected from the cylinder unburned.