Phenomena accompanying the self-ignition period have been and are 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 tests. 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 stands to those obtained from real engines raises a number of doubts. Because the self-ignition delay period determines the combustion process, I made a theoretical analysis of 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 the AVL LIST GmbH in Graz, Austria. The engine is equipped with Common Rail injection system. The test stand meets the standards: Directive 1999/96/EC of the European Parliament and the Council of 13th December 1999, Regulation (EC) No 715/2007 of the European Parliament and the Council of 20th June 2007, as well as Commission Regulation (EC) No692 /2008 of 18th 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 when fed 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.