## 1 Introduction
Lower calorific fuels require longer fuel injection times, while higher ones require the opposite. Longer injection times cause an incomplete combustion process, which increases the emission of carbon oxides, soot, and hydrocarbons in the exhaust gas. Another parameter determining the fuel injection properties is the injection advance angle. It is affected by the physical fuel properties, such as the density, kinematic viscosity, and volumetric coefficient of elasticity of the fuel. If the mentioned parameters increase, then the attenuation of fuel in the injector decreases, and it causes faster needle rising in the atomizer, which extends the injection advance angle [PERSON] et al. [11]. This results in increased pressure in the combustion chamber and a higher emission of nitrogen oxides. Density, viscosity and sound velocity value have an impact on the compressibility factor of the fuel during flow through the fuel atomizer holes, but they do not affect the Reynolds number [PERSON] et al. [12]. The fuel injection CFD simulation was carried out in two phases: Incompressible flow at constant liquid properties and compressible flow at liquid properties, calculated locally as a function of flow pressure conditions. Analyzing the research carried out by the authors, it can be stated that the issues related to fuel flow through the injector atomizer should be considered in terms of compressibility of the liquid, depending on the amount of pressure and flow temperature. At low system pressures, the flow is moderate. The liquid is in a state of transition between the laminar and turbulent flow Payri at el. [13]. The injection dose factor is then dependent on the Reynolds number. However, when the pressure in the system begins to increase, the flow passes into the turbulent phase, then the injection dose factor is independent of the Reynolds number [PERSON] et al. [14]. In work [PERSON] et al. [15], the process of injection of methyl esters of rapeseed oil mixed with diesel oil was compared in the proportions--5% bio-additive (B5), 30% bio-additive (B30) and pure methyl ester (B100). Studies have shown that the parameters of the fuel stream for B5 and B30 fuels are similar. The pure rapeseed oil methyl ester stream has a higher flow rate, due to the higher density, but similar momentum. The analysis of the fuel injector operation showed that its speed depends on the fuel used. The increased viscosity of the biofuel causes the needle to rise more slowly. This is very important when generating small doses of fuel, which depend mainly on the dynamics of the fuel injector. It was noticed that clean biofuel has a slightly larger penetration range, but a narrower angle of opening of the stream, which affects the dose of injected fuel [PERSON] et al. [16]. Due to the increased density, the speed of the injected fuel is lower, and the surface tension causes poorer atomization of the droplets. These factors cause worse mixing conditions with air and directly influence the combustion process. Biofuels have an inclination towards dampness absorption [PERSON] et al. [17] and [PERSON] et al. [18]. Research conducted by [PERSON] et al. [19] discussed the spray effects two fluids: Diesel and water, and rape oil and water. The results showed that the micro-explosion of drops in the biofuel stream needs more energy because of higher viscosity and surface tension compared to diesel fuel. The structure of primary and secondary breakup droplets depends on the Weber number [PERSON] et al. [20]. After analyzing the literature on the subject of this work, it can be stated that phenomena occurring during the course of fuel atomization, such as the formation of waves, their amplitude increase, and loss of stability, are affected by fluid vibrations during motion. The frequency of vibrations varies, and their causes can be external and internal.