The dissertation deals with the combustion efficiency of dual-biofuels (liquid and gaseous) in a compression ignition internal combustion engine, supplying the gas together with the intake air. The main object of research is the combustion process of dual-fuel, i.e., biogas and second-generation biodiesel, in a direct injection compression ignition engine. The goal of the dissertation is to determine the influence of these fuels on performance and emission indicators after experimental and theoretical research of dual-fuels, i.e., biodiesel and biogas, and to make suggestions on the efficiency of these fuels. The dissertation solves several main tasks: the dual-fuel influence on performance and emission parameters is researched using conventional diesel and alternative fuels (biodiesel, natural gas and biogas), changing the composition of the combustible mixture and the injection timing of liquid fuel. Based on the change of theoretical and mathematical modelling parameters of the compression ignition engine, the influence of dual-fuel on the combustion process was determined to evaluate the economic efficiency of dual-fuel use. The dissertation consists of an introduction, three chapters, a summary of the results, lists of used literature and the author’s publications on the topic of the dissertation. The introductory chapter discusses the research problem and the relevance of the work, describes the object of research, formulates the aim and objectives of the work, presents the research methodology, the scientific novelty of the work, the significance of the results of the work and the defended statements. At the end of the introduction, the author’s publications and conference presentations on the topic of the dissertation are presented, as well as the structure of the dissertation. The first chapter is assigned to a literature review. It looks into the factors promoting the use of dual-fuel, research methodology and results of the use of natural gas and biogas in internal combustion engines. The chapter conclusions are formulated, and the tasks of the dissertation are specified. The second chapter presents a methodology for investigating the use of dual-fuels in internal combustion engines. The third chapter presents the results of experimental studies and numerical modelling of the use of dual-fuel in a compression ignition engine.