## 3 Research Results and Discussion
### Environmental Indicators  
The carbon dioxide (CO\({}_{2}\)) contained in the exhaust gas was a product of full combustion which formed when there was enough oxygen for a full combustion process to take place. On the other hand, the CO\({}_{2}\) content increased with the increasing carbon content in the fuel molecule. Research of the chemical and physical properties of the fuel revealed that the carbon/hydrogen mass ratio in the D100 was 6.31, and it was 6.48 (2.7% higher) in the F100 (Table 2). The analysis of the key differences between diesel and chicken fat molecules distinguished a higher carbon content in the diesel molecules and the replacement of alkane chains--which form diesel hydrocarbons, for the most part--with chains of fatty acids that contain the isomeric branches in chicken fat. This accordingly affected CO\({}_{2}\) formation processes, where oxidation kinetics became faster with the formation of peroxide foci, and larger amounts of full combustion products, such as CO\({}_{2}\), formed during combustion [42]. This trend is also illustrated in the graph (in Figure 13), which shows that when increasing the chicken fat concentration in the mixtures at all engine loads, the CO\({}_{2}\) emissions tended to increase when the fat concentration exceeded 30%. Having replaced the diesel with pure fat, the CO\({}_{2}\) emissions increased by 4-5% when EGR was off. In addition to the above-listed reasons, the increase in CO\({}_{2}\) emissions was also affected by the rising fuel consumption (chicken fat consumes \(\sim\)16.3% more when EGR is off and consumes \(\sim\)18% more when EGR is on (Figure 9)). Fuel consumption increased as EGR reduced the BTE of the engine (Figure 12). With EGR on and the engine running on diesel, the F100 fuel's CO\({}_{2}\) emissions, compared with the D100, increased by 8-9% and by 11% and 12-15%, reaching 1100 g/kWh when increasing the fat concentration to 100% at a low engine load (BMEP = 0.3 MPa). Increasing the engine load to medium (BMEP = 0.6) decreased the BSFC (Figure 9) and reduced the CO\({}_{2}\) emissions. However, chicken fat can be considered a renewable fuel, and thus the CO\({}_{2}\) emissions decline when assessing the life cycle of fuel. Increasing the concentration of the fat in the fuel mixture increased its viscosity and injection quality, while a larger droplet size of the fuel mixture in the cylinder had an adverse impact on the combustion process, as it failed to evaporate, mix with compressed air and burn, resulting in an incomplete combustion product: carbon monoxide (CO). Given the ROHR (Figure 2), we see that increasing the concentration of the chicken fat in diesel decreased the heat release and combustion temperature in the premixed combustion phase, which resulted in a worse combustion process and an increase in CO emissions (Figure 14). However, the CO emissions were not high in the diesel engine due to excess air, and it ranged from 0.75 g/kWh to 3.5 g/kWh in the experimental research. When increasing the concentration of chicken fat in the fuel to 100%, the CO emissions increased by up to 30-80% under various loads with EGR off. With EGR on, the combustion process was even  
Figure 13: Dependence of CO\({}_{2}\) emissions on fuel composition and engine load. worse, resulting in higher CO emissions due to the reduced oxygen concentration in the cylinder (Figure 15) and further reduced combustion temperature (Figure 4). Compared with pure diesel, when EGR was off, the CO emissions increased to 57-88% when using pure fat with EGR off. This is in line with research on waste vegetable oil and waste animal oil-based biodiesel [25]. CO-specific emissions reduced due to a decrease in the BSFC, increasing the engine load from low (BMEP = 0.3 MPa) to medium (BMEP = 0.6 MPa), and the combustion temperature increased. The assessment of the fuel composition (Table 2) allowed for determining that the stoichiometric air/fuel ratio (kg/kg) for the D100 fuel was 14.75 kg and 12.60 kg for the F100 (14.5% less).