## 3 Research Results and Discussion
### Combustion Indicators  
Numerical analysis was conducted using AVL BOOST software with the BURN utility, running on different fuels with EGR off and on when \(n=2000\) rpm and BMEP = 0.4 MPa. Figure 2, which illustrates the rate of heat release during combustion, allows stating that the start of combustion (SOC) of all the fuels and their mixtures was \(\sim\)3 CAD after top dead center (ATDC). The ECU was used to set the start of injection (SOI) to \(\sim\)1 CAD before top dead center (BTDC) in this engine operating mode, which suggests that the \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Parameter, Units**} & \multirow{2}{*}{**Standard, Device**} & \multirow{2}{*}{**Accuracy**} & \multicolumn{2}{c}{**Fuel**} \\ \cline{1-1} \cline{4-5}  & & & **D100** & **F100** \\ \hline Gross heating value, MJ/kg & LST CEN/TS 14918:2006 & \multirow{2}{*}{0.13 MJ/kg} & 45.3 & 39.3 \\ Lower heating value, MJ/kg & IKA C 5000 calorimeter & & 42.5 & 37.6 \\ \hline \multirow{2}{*}{Flash point, \({}^{\circ}\)C} & ISO 2719:2013 & \multirow{2}{*}{0.03 \({}^{\circ}\)C} & 67.1 & 120 \\  & FP93 5G2 Pensky-Martens analyzer & & & & \\ \hline Kinematic viscosity at 40 \({}^{\circ}\)C, mm\({}^{2}\)/s & ASTM D7042, Anton Paar SVM 3000/G2 & 0.1\% & 2.95 & 33.8 \\ Density at 40 \({}^{\circ}\)C, kg/m\({}^{3}\) & Stabinger viscometer & 0.0002 g/cm\({}^{3}\) & 819.8 & 880.8 \\ \hline Oxidative stability, min. & EN 16091, PetroOXY analyzer & 0.1\% & 56.6 & 15.5 \\ \hline CFPP, \({}^{\circ}\)C & EN116; ASTMD6371, FPP 5 Gs analyzer & 1 \({}^{\circ}\)C & \(-\)22 & 26 \\ \hline WSD, \(\mu\)m & ISO 12156-1:2006, high-frequency & \multirow{2}{*}{1 \(\mu\)m} & \multirow{2}{*}{404} & \multirow{2}{*}{271} \\  & reciprocating rig method & & & \\ \hline Water content, \% mass & ISO 12156-1:2006, & \multirow{2}{*}{0.0003\%} & \multirow{2}{*}{0.004} & \multirow{2}{*}{0.153} \\  & Aquamax KF Coulometric analyzer & & & \\ \hline Sulphur, \% mass & IKA C 5000 calorimeter, muffle furnace, scales & 0.04\% & \multirow{2}{*}{\
\begin{tabular}{} \end{tabular} } \\ \hline Element composition & C, \% & \multirow{2}{*}{
\begin{tabular}{} \end{tabular} } \\ H, \% & & & \\ \hline C/H ratio & & & 6.31 & 6.48 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Fuel properties. duration of the ignition delay phase of all the fuels was similar and was about 4 CAD. In the premixed combustion phase, the maximum ROHR value was achieved using pure diesel (EGR Off = 37.6 J/CAD at 9 CAD ATDC, and EGR On = 30.0 J/CAD (a decrease of \(\sim\)21%) at 9 CAD ATDC). Having increased the chicken fat concentration to 30%, 50%, 70% and 100% in the premixed combustion phase with EGR being off, the ROHR decreased by \(\sim\)11%, \(\sim\)17%, \(\sim\)21% and \(\sim\)27%, respectively, and with EGR being on, the ROHR decreased by \(\sim\)12%, \(\sim\)16%, \(\sim\)17% and \(\sim\)22%, respectively. The increase in chicken fat concentration in the fuel was observed to shift the ROHR to a later combustion, and the diffusion phase was more intensive. With the engine running on diesel, the maximum ROHR value in the diffusion phase was at 29.5 J/CAD with EGR off and 28.1 J/CAD with EGR on. After increasing the chicken fat concentration to 100%, the maximum ROHR of the diffusion phase increased up to \(\sim\)8% with EGR off and up to \(\sim\)5% with EGR on. Animal fat contains more heavy compounds, which do not evaporate or burn easily. Thus, combustion takes place at a later phase. Chicken fat injected in the cylinder did not evaporate easily, and the fuel started burning later compared with diesel due to a higher molecular weight and the tension film strength, which was also confirmed by [39; 40]. More time was needed for the chicken fat droplets to evaporate and burn. Thus, the peak ROHR was reached at a later combustion phase. A lower ROHR reduces the intensity of thermal NOx formation in the premixed combustion phase, while a later combustion improves soot oxidation, but has a negative impact on engine energy efficiency [41].