*4.3.2 Carbon dioxide (CO2) emission*

Unlike CO emission, CO2 emission is considered as the primary product during combustion; and its presence in exhaust gas signifies the completion of fuel's combustion inside the engine cylinder. Again, concentration of CO2 emission is influenced by the fuel's molecular properties like unsaturation, C/H ratio, and even aromaticity; besides the operating condition of the engine [46]. From **Table 3** and **Figure 7**, both blend and ester samples reported higher CO2 emission than diesel sample citing the presence of their fuel bound oxygen molecules and their higher cetane number; which prolonged its combustion duration for their complete oxidation. Equally important, higher concentration of CO2 emission was also contributed by the long carbon chains in their FAE molecules. Accordingly, B10 blend reported 23.59%, B20 blend reported 35.76% and B30 blend reported 45.58%, higher CO2 emission than compared to neat diesel. Likewise, stearate and palmitate blends reported increased CO2 emission by 6.23% and 15.41% higher CO2 emission, respectively; meanwhile, oleate blend reported lower CO2 emission by 6.97%.

Upon comparing ester samples with Biodiesel (B20) blend, palmitate blend reported 12.33%, stearate blend reported 18.72%, and oleate blend reported 28.32%, lower CO2 emission. As a matter of fact, both palmitate and stearate blends reported

**Figure 7.** *Carbon dioxide emission of WaFO B20 blend and ester samples.*

higher CO2 concentration than oleate blend on account of their reduced availability and saturation, which improvised their overall effectiveness of combustion. Especially, palmitate blend reported its CO2 emission closer to B20 blend, stating its higher concentration than stearate blend; and its ability to initiate early combustion, thereby providing sufficient time for the accumulated diesel to combust completely. Meanwhile, oleate blend reported lowest CO2 emission amongst all test samples on account of its unsaturation and increased availability, which reduced the effectivity of atomization thereby combusting poorly [48]. In addition, delayed SOC allowed it to combust rapidly which hindered its complete oxidation, thereby forming incomplete combustion products. Summing up, higher concentration of CO2 emission for WaFO biodiesel, inspite of its unsaturation was explained by the presence of its saturated FAEs (ethyl palmitate and ethyl stearate), which initiated early SOC and ensured the progress of combustion of the unsaturated FAEs (ethyl oleate). Again, CO2 emissions increased along with engine load, and were also explained by the increasing amount of fuel injected into the cylinder to meet the energy demand of the engine.
