*4.3.3 Nitrogen oxide (NOX) emission*

Often, NOX emission in exhaust gas is also regarded as secondary by-product during combustion; however, it arises when engine reports high operating temperatures, especially high exhaust gas temperatures. In relevance to that, NOX emissions due to high EGTs are explained by higher cetane number and fuel bound oxygen content inducing prolonged combustion; besides the viscosity of fuel [30, 32]. From **Table 3** and **Figure 8**, both blend and ester samples reported higher NOx emission due to their shortened ID, and increased viscosity; which increased the overall duration of combustion, and liberate sufficient heat energy fairly enough for producing NOX emission. In

*Molecular Contribution of Fatty Acid Esters in Biodiesel Fueled CI Engines DOI: http://dx.doi.org/10.5772/intechopen.102956*

**Figure 8.** *Nitrogen oxide emission of WaFO B20 blend and ester samples.*

addition, calorific value of these test samples also contributed to this harmful emission. Supporting this, NOX emission was increased by 22.17% for B10 blend, 28.2% for B20 blend and 36.06% for B30 blend; and 6.55% for stearate blend, 11.8% for palmitate blend, and 17.3% for oleate blend, than compared to neat diesel.

In comparison with B20 (biodiesel) blend, oleate blend reported 8.53%, palmitate reported 12.66% and stearate blend reported 16.57%, lower NOX emission. In specific, palmitic and stearate blend reported lower NOX emission than B20 blend signifying their early SOC due to shortened ID; and provided sufficient activation energy for initiating the combustion of diesel during diffusion combustion phase. Yet, these samples reported reduced NOX emission because of their volatility. On the other hand, oleate blend exhibited higher NOX emission owing to its increased availability, high viscosity, and reduced cetane number which led to its accumulation in event of its delayed SOC. Besides, rapid combustion of this accumulated fuel liberated high temperature inside the cylinder, and produced high NOX emission. Outlining these results, higher NOX emission of WaFO biodiesel was influenced by its unsaturated FAEs (ethyl oleate), which liberated very high temperatures inside the cylinder, thereby forming high NOX emissions. Interestingly, saturated FAEs (ethyl palmitate and ethyl stearate) also liberated very high temperatures during premixed phase, besides contributing activation energies to unsaturated FAEs, thus favoring NOX formation. Like other emissions, NOX emissions of both blend and ester samples increased with engine load on account of more fuel being combusted inside the engine to meet the energy demand, thereby delivering their equivalent work and heat.

### *4.3.4 Exhaust gas temperatures (EGT)*

EGT from the engine defines the progress of combustion inside the cylinder; and is dependent on the engine's operating conditions and properties of fuel used.

Conventionally, fuel reporting delayed SOC, with prolonged duration exhibits higher EGTs; and these high temperatures contribute to NOx emissions [49, 50]. From **Table 3** and **Figure 9**, both blend and ester samples exhibited higher EGTs accounting their higher cetane number and fuel bound oxygen content; which favored higher rate of combustion and liberated large amount of heat. Moreover, viscosity and calorific value of these samples also contributed to their high EGTs. Supporting this, B10 blend reported 20.97%, B20 blend reported 26.49% and B30 blend reported 31.52%; and stearate blend reported 6.1%, palmitate blend reported 10.62%, and oleate blend reported 15.46%, higher EGTs than compared to neat diesel.

Amongst ester samples compared with B20 biodiesel blend, oleate blend reported 8.48%, palmitate blend reported 12.23% and stearate blend reported 15.76%, lower exhaust gas temperature. Especially, palmitate and stearate blends combusted earlier due to shortened ID, which forced the highly volatile, accumulated diesel to combust rapidly, and limiting the heat generation and EGT. In case of oleate blend, low cetane number allowed it to undergo prolonged combustion, and assisted the diesel for combustion during diffused combustion and after burning phase; thus liberating large amount of heat and increase its EGT. On the whole, WaFO biodiesel with significant amount of unsaturated FAEs (ethyl oleate) exhibited prolonged combustion accompanied with high rate of combustion using their fuel bound oxygen molecules; and liberated high EGTs [29]. Meanwhile, saturated FAEs (ethyl palmitate and ethyl stearate) contributed to a minimal amount to EGT, accounting their early ignition and supplying of activation energy to the unsaturated FAEs; thus contributing minimal to EGTs. Like NOx emissions, EGT of both blend and ester samples increased with engine load on account of more fuel being combusted inside the engine to meet the energy demand, thereby delivering their equivalent work and heat.

**Figure 9.** *Exhaust gas temperature of WaFO B20 blend and ester samples.*

*Molecular Contribution of Fatty Acid Esters in Biodiesel Fueled CI Engines DOI: http://dx.doi.org/10.5772/intechopen.102956*

#### *4.3.5 Unburnt hydrocarbon (HC) emission*

Unburnt hydrocarbons in exhaust gas signifies the inability of the fuel to get completely combusted near the cylinder wall, owing to reduced flame temperatures near the fuel-rich zones and poor combustion kinetics and quenched flame [51]. From **Table 3** and **Figure 10**, both blend and ester samples displayed reduced HC emission in event of complete oxidation using their fuel bound oxygen content. Adding to this, these oxygen molecules helped in liberating high flame temperatures, and propagated throughout the cylinder and combusted unburnt hydrocarbons. Unfortunately, neat diesel reported traces of HC emission as a consequence of its rapid combustion, owing to its high volatility which reduced the adiabatic flame temperature near the cylinder walls. In comparison with diesel blend, B10 blend reported 17.75%, B20 blend reported 10.91% and B30 blend reported 5.89%; and oleate blend reported 23.71%, palmitate blend reported 27.15%, and stearate blend reported 34.72%, lower HC emissions.

Relatively, stearate blend reported lowest HC emission (by 27.22%), followed by palmitate blend (18.54%) and oleate blend (14.66%), than compared to B20 (biodiesel) blend. Supporting this, palmitate and stearate blends exhibited lower HC emission, and was clearly evident that presence of oxygen in these samples reduced their HC emission. Explaining this, these saturated FAEs initiated early combustion and provided sufficient temperature inside the cylinder for ensuring complete oxidation of diesel [27]. Whilst, oleate blend reported higher HC emission than other ester samples because of its increased availability and unsaturation; which resulted in poor atomization and vaporization, and reduced the effectiveness of combustion (i.e. in complete combustion) for the liquid droplets present at the localized zone with reduced flame temperatures. Consolidating these results, it can be concluded that WaFO biodiesel

**Figure 10.** *Hydrocarbon emission of WaFO B20 blend and ester samples.*

combusted completely using its fuel bound oxygen content. Interestingly, unsaturated FAEs (ethyl oleate) in WaFO biodiesel reduced its rate of atomization, thus forming micro fuel droplets; however, they were combusted by the heat energy supplied by the saturated FAEs (ethyl palmitate and ethyl stearate). Here, HC emission of test samples increased with engine load; yet, HC emission of blend and ester samples remained lower than diesel sample even at higher loads due to high engine temperatures, besides their high cylinder pressures [52].
