The development and research works on liquid fuel injection in spark-ignited direct injection (SIDI) engines, apart from so common in recent years simulation methods, still have a significant cognitive substrate. This is related to experimental research on repeatability of combustion process using multi- and mono-cylinder test engines and Rapid Compression Machines. The repeatability of preparation and delivery processes has immediate impact on repeatability of combustion process. Except for the necessity of obtaining the repeatability of fuel amounts, the repeatability of injected fuel spray is required. The penetration range and spray area in combustion chamber have direct impact on mixture creation and formation. The optical research on fuel injection has been made in order to determinate its repeatability. The research on unrepeatability of fuel spray propagation has been conducted using piezoelectric injectors of outward-opening type, being primary elements of the spray-guided combustion systems. The results of research were presented in the form of index of variation of the selected parameters. The evaluation of the results of the optical research concerns radial spray penetration and fuel spray velocity. Unrepeatability has been presented with coefficient of variation of radial penetration in relation to the time of injection duration. It has been observed that the coefficients of various parameters are lower with longer times of fuel injection.