## 3 Properties of the Mixture of Jet A-1 Aviation Kerosene and FAME Biofuel  
The used biofuel FAME is a fatty acid methyl ester of rapeseed, which is commonly used in the food industry. Crude rapeseed oil is produced by transesterification, a chemical process based on mixing methanol with oil pressed from rapeseeds. Sodium hydroxide acts as a catalyst during the mixing process. The characteristics of obtained FAME biofuel can be seen in Table 2. The by-product of this process is glycerin, which is used in the chemical, cosmetic, and pharmaceutical industries [31; 32; 33]. Figure 3: Small jet engine lab. Jet A-1 aviation kerosene is the most widespread aviation fuel used in aviation jet engines (see characteristics in Table 3). Jet A-1 is a mixture of hydrocarbons with a boiling point of 130-300 \({}^{\circ}\)C. It consists of light fractions with a lower distillation temperature and lower density than light diesel fuel. Various additives are added to aviation kerosene to improve the necessary properties, e.g., antioxidants, conductivity additives, and additives to increase the lubricating ability. Aviation kerosene must be hydrogenated. Jet A-1 has a freezing point determined by Boeing in document 747-SL-28-68 of at least \(-\)47 \({}^{\circ}\)C [34; 35]. jet kerosene contains additives that increase its resistance to bacteria, it is assumed that different concentrations of FAME biofuel and Jet A-1 jet kerosene blends will be more resistant to bacteria than pure FAME biofuel. The physical properties of FAME biofuel and Jet A-1 (PL-7) aviation kerosene are very similar (Table 4). The Influence of the Mixture of FAME Biofuel and Jet A-1 Aviation Kerosene on the Operating Characteristics of Jet Engines  
The effect of different blends of FAME biofuel and Jet A-1 aviation kerosene on the operating characteristics of jet engines was experimentally verified in the laboratory of small jet engines on the experimental small jet engine SJE-20. The experimental engine was not specially modified for this type of fuel (it was not modified in terms of its design and control system). The individual experimental measurements used different mixtures of FAME biofuel and Jet A-1 aviation kerosene. Prior to each measurement, a blend was prepared with the appropriate addition of FAME biofuel to Jet A-1 aviation kerosene. The individual blends differed in the percentage of FAME biofuel in aviation kerosene. This resulted in 11 different blends of FAME biofuel and Jet A-1 aviation kerosene, as shown in Table 5.  
The measurement cycle was the same for all the mixtures used and consisted of starting a small jet engine (the start-up cycle was completed at around 15 s) and the collection of the measured values of the selected parameters at selected times of 20 s, 25 s, 30 s, 35 s, 40 s, and 45 s. The given times from the start of the measurement were selected based on the steady-state operation of the motor after the start-up transient. Each measurement (up to 40% FAME share in Jet A-1 aviation kerosene) was repeated at least five times, with a measurement length from 40 s to 55 s. At higher proportions of FAME than 40% in the mixture with jet A-1, the number of measurements was increased due to unsuccessful attempts with a cold SJE-20 engine. Thus, experiments with a higher FAME content than 40% were carried out only when the engine was warmed up. The consumption of the mixture ranged from 0.5 dm\({}^{3}\) to 0.65 dm\({}^{3}\). At each experimental measurement, atmospheric temperature t\({}_{0}\), atmospheric pressure p\({}_{0}\), and humidity were measured. As the measures took place in an air-conditioned laboratory, the temperature was constant t\({}_{0}\) = 21 \({}^{\circ}\)C, the humidity was 75%, and the treasure varied in the p0 = 101,328 to 101,650 Pa range. During the measurements, the following parameters were continuously measured: the static air temperature in front of the engine