The condition for meeting the current requirements of compression ignition (CI) engine users is that the combustion processes should be carried out properly. The analysis of these processes for cognitive purposes is very difficult due to the simultaneous combustion and fuel injection processes, the phenomena of fuel injection and atomization from the air, evaporation of fuel droplets, chemical reactions and proper combustion process. In addition, we deal with with heterogeneity of the

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... e mixture. In different parts of the combustion chamber, there are drops of liquid fuel alone, a mixture of fuel vapors with air and air only. During the entire combustion process in the chamber space, the excess air ratio has various values in different areas of its volume. Self-ignition occurs most often when λ = 0.8-0.9 and then the flame front spreads [19] . Self-ignition of hydrocarbons occurs after fuel injection into the combustion chamber. It is particularly important for the dynamics of the combustion process. In a CI engine, there is a short period of time to mix the injected fuel with the compressed air in the engine cylinder. Therefore, it is necessary to create such conditions where oxygen and atomized fuel droplets are close, which are prerequisite for fast ignition. The self-ignition delay process is influenced by a number of factors from the group of: chemical (mainly fuels), physical (external environment), construction (engine design) and operational (engine operation AbstrAct The phenomena accompanying the self-ignition period have been the subject of extensive research in this area. They are usually carried out in constant volume pressure chambers or in reactors with constant air flow. Such tests are considered to be basic. The conditions in these tests are definitely different from those in compression ignition engines. Therefore, the comparison of the auto-ignition delay periods from test setups to those obtained from real engines raises a number of doubts. Because the self-ignition delay period determines the combustion process, a theoretical analysis was conducted, pertaining to a number of factors that have an impact on this process which determines the operational aspects of the engine, and thus its economics and ecology. The research object was a single-cylinder engine from AVL LIST GmbH in Graz, Austria. The engine is equipped with a Common Rail injection system. The test setup meets the following standards: Directive 1999/96/EC of the European Parliament and the Council of 13 th December 1999, Regulation (EC) No 715/2007 of the European Parliament and the Council of 20 th June 2007, as well as Commission Regulation (EC) No 692 /2008 of 18 th July 2008. The analysis of the operational aspects of the self-ignition delay period was based on the results of tests on the AVL 5402 engine fueled with hydrocarbon fuels: diesel and synthetic oil. The engine was also fed with vegetable fuel -rapeseed oil. The obtained material from the tests warns the user of CI engines against the effects of their failure if the engine control parameters and the quality of fuel for its supply are not maintained as recommended by the manufacturers. The material contained in the publication is used for scientific analysis, and, which is worth emphasizing, is of a utilitarian nature.