In this final paper, fuel injection systems of the Diesel engines are described and analysed. First and foremost, the principle of operation of Diesel engines through four strokes is displayed. It also contains the main difference compared to the Otto-engine, which is that with Diesel engines the fuel is being injected into the already compressed air in the cylinder, while discussing the Otto-engine, a previously created mixture of fuel and air is injected into the cylinder. Given that the fuel injection system is one of the most important steps regarding the development of Diesel engines, the next chapter contains a detailed process of the injection and combustion gas in the cylinder of this type of engine. The main task of each injection system is to obtain fuel from the tank, then compact it on a certain pressure and finely disperse in the combustion space in order to achieve complete combustion of the mixture. To achieve complete combustion, i. e. soft and proper operation of engine, it is necessary to synchronize the work of all components of the injection system to avoid malfunction of the motor itself. In addition, the paper presents two performances of Diesel engines, that is, two types of fuel injection. The development of high-pressure pumps and injectors throughout history helps in achieving a high injection pressures that result in large loads of the drive mechanism. Because of this disadvantage, the engines with indirect injection, that is, with different performances of the antechamber are applied. Engines with indirect injection had called for the installation of a cool start device which created additional costs and problems. By introducing the fuel pre-injection into the fuel injection process itself, a problem of high combustion pressures is fixed and the engines with direct injection are back in use, which due to its characteristics, have become irreplaceable. The last chapter analyses the different fuel injection systems, from old mechanically controlled high-pressure pumps all the way up to modern electronic regulated Common rail system, and the work of their main components.