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Mathematical modeling of the working cycle of a piston engine is a highly complex process, as it involves thermodynamic and gas-dynamic phenomena occurring within a variable-volume cylinder, taking into account combustion chemistry and charge exchange processes. The co-combustion of fuels with different reactivities is a process that occurs, among others, in a dual-fuel compression ignition piston engine. Such a combustion system is determined by numerous physical and chemical factors. In this system, there is port fuel injection (PFI) of a low-reactivity fuel (C2H5OH) and direct injection into the combustion chamber of a high-reactivity fuel, such as diesel. In the first phase, the quality of the prepared combustible mixture in both cases is determined by physical phenomena, such as the range and shape of the fuel spray, atomization, evaporation of fuel droplets, and the mixing process with air, supported by flow phenomena. PFI and direct injection differ physically because they occur under different gas-dynamic and thermal conditions. Once a combustible mixture is obtained, its ignition is controlled by combustion kinetics. The kinetics of chemical phenomena are described by combustion models that account for high-temperature oxidation processes. This paper presents the results of CFD simulation studies of a dual-fuel piston engine with two independent injection systems. The simulation analysis focused on the effect of the type of ethanol injector on the distribution and evaporation of the fuel during the cylinder filling stroke. The analysis included the rate of heat release and emissions of toxic exhaust components. The model's sensitivity to changes in fuel ratios and combustion process control was confirmed by experimental results for selected engine operating points.