The undesirable deposits forming on the surfaces of various internal parts of reciprocating internal combustion engines and the systems operating in conjunction with them worsen during the operation of the engines and threaten their proper functioning. The deposits form as a normal result of the processes of fuel injection and creating and combusting the fuel–air mixture in engines. It was not investigated until the beginning of the 21st century, when extensive multi-directional research began not only to identify the causes of these deposits, the mechanisms behind their formation, and the factors leading to deposit growth, but also to determine the chemical composition of various groups of deposits. Such research became necessary because engines must comply with gradually tightening regulations on environmental protection, necessitating the introduction of increasingly complex engine designs and strategies for controlling the processes of precise and divided fuel injection into the combustion chambers and advanced algorithms for controlling the combustion processes according to the combustion system and the purpose of the engine. However, it became apparent that the co-functioning of the increasingly complex engine technologies and solutions, particularly of fuel injection systems, may be significantly disturbed by the deposits forming inside them. More and more complicated engine designs with tighter and tighter tolerances of the working parts necessitate the multi-directional testing of harmful deposits. An increasing number of factors affecting deposit formation are being identified, which leads to the development of increasingly complex classifications and subdivisions of deposits according to their type, composition, and form. At the same time, the search for lower emissions and greater engine efficiency is driving further mechanical changes in engines and vehicles. The higher temperatures and pressures connected with these changes are likely to impact the fuel being handled within the fuel and combustion systems. Such effects will inevitably cause the deposit chemistry and morphology to change. The size of the coke deposits produced may disturb the processes of fuel atomization, of filling the engine combustion chambers and swirling the charge, and in consequence may affect the efficiency of filling and the quality of the fuel–air mixture. These problems led to the development of a number of standardized and unstandardized methods for assessing the size of deposits. It was found that in the case of SI engines, the deposits that most endanger correct engine operation are those which are formed in the combustion chambers, on the inlet valves, inlet ducts, and fuel injector tips. The most common sign of deterioration caused by deposits is the loss over time of the performance, usability, and operational value which were originally declared by the manufacturer. In the case of CI engines, the most dangerous are coke (carbon) deposits formed on the external surfaces of the fuel injector nozzle tips and inside the injector nozzle orifices. In Europe, mandatory procedures for assessing the size of different coke deposits formed on different components in both SI and CI engines are being developed by the Coordinating European Council for the Development of Performance Tests for Transportation Fuels, Lubricants, and Other Fluids (CEC). The theoretical part of this publication reports the problems of the deposits produced in reciprocating internal combustion engines and their fuel systems. It discusses standard and non-standard engine test methods for both quantitative and qualitative assessment of deposits and presents the significance of the assessment methods which are currently used for the classification of deposits. The publication also presents the scope of application and the usefulness of methods for determining the threats posed to the functioning of an engine by various types of deposits and methods for identifying the causes of deposit formation, in particular those related to the composition of the fuels and lubricating oils used. The effects which fuel composition and the engine’s construction and operating parameters have on various engine deposits, the possible causes of deposit formation, and the importance of modern deposit control additives and high-technology solutions in counteracting this detrimental phenomenon are also all discussed. The experimental part presents the results of research carried out at the Oil and Gas Institute – National Research Institute concerning: • the incomparability of measurements of fuel performance obtained from various engine tests, • studies on the influence of various deposit control additives on the formation of harmful engine deposits during engine tests, • the influence of fuel treatments on the deposit formation processes in internal combustion engines (described qualitatively or quantitatively), • determination of the impact which various chemical compounds, serving as contaminants within the fuels, have on deposit formation in internal combustion engines and fuel injection systems, • determination of the impact that various chemical structures of the compounds within the fuels and biofuel blends have on deposit formation in internal combustion engines and fuel injection systems, • studies on the influence of bio-components contained in both petrol and diesel fuels on tendency for deposits to form in internal combustion engines, and • multidirectional studies on the impact of FAME degradation processes in biodiesel fuel blends on the formation of harmful engine deposits.
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