## 3 Research Results and Discussion
### Environmental Indicators
The lower required air mass was due to the oxygen (O\({}_{2}\)) contained in the fuel. Increasing the engine load to medium (BMEP = 0.6 MPa) increased the hourly fuel consumption but decreased the BSFC (Figure 9) as the BTE increased (Figure 12). However, the BSFC increased at different engine loads, and having replaced the D100 with F100, it increased by \(\sim\)16 when EGR was off and \(\sim\)18% when EGR was on. This is why the O\({}_{2}\) concentration in the exhaust gas decreased somewhat by 1.7-2.5% with EGR off and when the fuel contained 50-70% chicken fat (Figure 15). With EGR on, the O\({}_{2}\) concentration in the exhaust gas decreased by 21-28% at different loads due to a lower intake of air mass. When using chicken fat, the O\({}_{2}\) concentration declined by an additional 1.4-2.5%. However, the O\({}_{2}\) concentration was observed to slightly increase when the chicken fat concentration was 100%. This was due to the exhaust energy being intensified by a longer combustion process,  
Figure 14: Dependence of CO emissions on fuel composition and engine load. Figure 15: Dependence of the O\({}_{2}\) concentration on fuel composition and the engine load. which increased the turbocharger pressure (Figure 8). The engine volumetric efficiency depended on the turbocharger pressure. A higher fuel C/H ratio, as well as the decreasing O\({}_{2}\) concentration in the exhaust gases, suggests that an increase in the chicken fat concentration in the fuel increases the opacity of exhaust gases. However, the experimental research results were the opposite; at different engine loads and with EGR being off, increasing the chicken fat concentration from 0% to 100% led to a gradual decline in the smoke by 57-70% (Figure 16). With EGR on, a lower excess air and lower combustion temperature resulted in an increase in the smoke opacity of the D100 fuel by 2.5-3.3 times compared with when EGR was off. When using the D70F30 fuel mixture, the smoke decreased by 16-23%, and when using the F100, the smoke decreased by as much as 2.3-3.0 times compared with the D100 when EGR was on. Increasing the concentration of the chicken fat decreased the smoke due to a later combustion process. Soot burns better in the diffusion phase with a more intense ROHR and higher combustion temperature compared with D100. Oxygen contained in the chicken fat also contributes to the better combustion of soot. The compression ignition engine operated in the presence of excess air and, therefore, the concentration of unburnt hydrocarbons (HCs) in the exhaust gas was low. With the engine running at different loads, increasing the chicken fat concentration in the fuel to 50% led to a slight downward trend in HC emissions, which decreased to 6% when EGR was off (Figure 17). However, when increasing the chicken fat concentration above 50%, the HC emissions started increasing again, which likely was due to the deteriorating quality of the injection due to the increasing fuel viscosity. The chicken fat also had a similar impact on HC emissions with EGR being on. The combustion temperature and oxygen that was not used during combustion had a major impact on the nitrogen oxide (NO\({}_{\mathrm{x}}\)) emissions. The experimental research revealed that when increasing the concentration of the chicken fat in the fuel, the NO\({}_{\mathrm{x}}\) emissions gradually decreased in the engine throughout the entire research range. Having replaced the D100 with F100, with EGR off, the NO\({}_{\mathrm{x}}\) emissions were reduced by 13-22% at various loads (Figure 18). This can be explained by a lower ROHR intensity, a lower rise in temperature and the cylinder temperature in the premix phase (Figure 2a). In their experimental research on the use of vegetable oils and animal fats in compression ignition engines, [PERSON] and [PERSON] [25] observed the opposite change and stated that more nitrogen atoms reacted with oxygen, leading to higher NO\({}_{\mathrm{x}}\) emissions due to a high oxygen content.