## 3 Results and Discussions
### Exergy Analysis
The irreversibility in a system results in entropy generation, which is directly related to the exergy destruction rate. At the evaluated engine condition, the entropy generation of diesel, E15B35D50, and E15B35 FTD50 were respectively 0.0107, 0.0109, and 0.0110 kW/K (as shown in Figure 6). This could be partially explained by the higher mechanical friction at lower engine loads, as previously reported in [69], which is the major thermodynamic irreversibility of combustion engines. However, the friction losses were not covered within this work. Moreover, it has been reported that higher fuel consumption, as well as the combustion process itself, lead to more irreversibilities [27]. [PERSON] et al. [32] have reported that the exergy destruction rate was higher with diesel fuel than with \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Fuel**} & **Fuel Exergy** & \multirow{2}{*}{**Work Rate (kW)**} & **Exhaust Gas Exergy** & **Heat Losses** & **Exergy Destruction** \\  & **Rate (kW)** & & **Rate (kW)** & **Rate (kW)** & **Rate (kW)** \\ \hline Diesel & 5.01 & 1.25 & 0.28 & 0.29 & 3.20 \\ E15B35D50 & 5.08 & 1.25 & 0.26 & 0.32 & 3.25 \\ E15B35 FTD50 & 5.10 & 1.25 & 0.27 & 0.30 & 3.29 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Exergy distribution in the engine control volume. diesel/biodiesel/ethanol blends, and thus the entropy generated increased. However, [PERSON] and [PERSON] [26] have found higher exergy destruction rates for 8-25% (\(\
u/\
u\)) of biofuels (biodiesel/ethanol) blended with a diesel under the same experimental engine operating condition, when compared to reference diesel. This can be explained by the difference between the injection systems (common-rail or conventional injection system) used in this research, which affects the timing that the fuel is injected, as it varies with the compressibility of the fuel, and the ignition time, which varies with the cetane number of the fuel blend. Furthermore, as shown in Equations (11) and (12), the term \(\varphi\) is directly proportional to the loss (i.e., higher \(\varphi\) leads to higher losses) and \(\varphi\) is higher in the blends, because of the increased fuel mass fractions of \(c\), \(h\), and \(o\). The sustainability index is directly proportional to the exergy efficiency of the control volume. It has been observed that the diesel fuel presented more sustainable characteristics in comparison to the E15B35 FTD50 blend. The SI values presented in Figure 6 were calculated to be 1.331 for diesel fuel, 1.325 for E15B35D50, and 1.323 for E15B35 FTD50. It has been previously reported by [PERSON] [71] that that pure diesel had higher SI than biodiesel. The researcher discussed that the SI is directly related to the exergy efficiency and that this efficiency was higher for the diesel than for the biodiesel because the LHV of the diesel was greater than that of biodiesel. Similar to the energy analysis, the exergy balance of each tested fuel was determined based on the inlet fuel exergy. Thus, the exergy balance of diesel, E15B35D50, and E15B35 FTD50 are shown in Figure 7. The graph shows that 24.4% of the available inlet fuel exergy of E15B35 FTD50 was used in the form of work, 5.3%, was discharged within the exhaust gases, 5.8% was computed as energy losses, while 63.8% was destroyed. It is clear that the exergy destruction rate represents the major fraction of the energetic balance in the combustion engine, which is due to the irreversibilities. However, it is possible to use part of the exergy from the exhaust gases. It has been previously recognized in the literature that combustion is the major source of irreversibility inside the ICE cylinder [47; 72]. [PERSON] et al. [69] stated that the fuel type does not show an apparent effect on the exergy losses or irreversibilities in the engine. The researchers argued that this could be due to the many variables that affect the irreversibilities.