## 1 Introduction
The numerical research revealed that the use of such mixtures was possible in a diesel engine, and the correspondence of results according to the R\({}^{2}\) values varied for different pollutants (most accurately for CO (0.93) and least accurately for CO\({}_{2}\) and NO\({}_{\rm x}\) (0.4 and 0.41, respectively)) [27]. When using chicken fat as fuel, several aspects to be assessed can be distinguished [28], including the combustion process, exhaust emissions and energy parameters. When it comes to the combustion process, the use of chicken fat and its mixtures with diesel led to a reduced rate of heat release due to the cooling effect of fuel evaporation. At a 100% load, the rate of heat release (ROHR) reduced from 70 J/degree (diesel) to 63 J/degree (25% chicken fat content in diesel). The same trend was observed in the case of the maximum in-cylinder pressure, measured as 7.0 MPa (diesel) and 6.8 MPa (25% chicken fat in diesel) [29]. It was also observed that the homogeneous charge compression ignition (HCCI) engine effect (where combustion is achieved at a temperature below the NO\({}_{\rm x}\) formation temperature) can be achieved when using chicken fat. Thus, the author suggested using chicken fat for low and partial loads only. Having increased the load, the engine worked as a normal compression ignition engine [30]. Increasing the content of the chicken fat in diesel led to a decrease in the exhaust gas temperature. Larger fuel droplets and their lower acceleration caused the temperature decrease [31]. The use of chicken fat resulted in lower NO\({}_{\rm x}\) emissions compared with diesel fuel, both during their generation [32] and combustion [33]. A noticeable reduction in CO and HC emissions compared with conventional diesel [34] and higher CO\({}_{2}\) emissions were observed, due to higher oxygen concentrations in the fat and diesel mixture [35]. Like all types of biodiesel presented in this reference work, chicken fat reduces PM emissions; in separate tests, PM emissions could decrease up to 65% compared with diesel [36]. Analysis of the energy aspects of the use of chicken fat revealed a lower brake thermal efficiency (BTE) indicator as a result of a higher fat viscosity, changed combustion process and the previously described higher brake specific fuel consumption (BSFC), forming for the same reasons as in case of the BTE [29]. The same trend was observed when analyzingturkey fat and diesel mixtures in a compression ignition engine [24]. Engine power using chicken fat also decreases, depending on the fat content in the fuel mixtures [37]. In summary, it can be concluded that there are relatively few studies on the indicators of compression ignition engines running on mixtures of chicken fat and diesel compared with other alternative fuels (e.g., biodiesel and hydrogen). These studies were observed to not use the exhaust gas recirculation (EGR) system. Therefore, research of the properties of diesel and chicken fat mixtures and analysis of the combustion process of compression ignition engines, as well as their environmental and energy indicators with EGR on and off, would allow for the expansion of the scientific area of application of these fuel mixtures.