This issue of the journal Thermal Science contains 27 papers dedicated to the research in the field of internal combustion engines. The attention of the researchers is mostly oriented to the global problems facing our planet, whose solution is crucial for future sustainable technological development. These are of course, on the one hand, energy efficiency and problems in finding and using alternative and preferably renewable fuels, and, on the other hand, environmental pollution. As these problems are highly related to internal combustion engines, research efforts are being focused in that direction. Research methods range from the application of different levels of mathematical simulation of engine working process to experimental investigation of engine global brake parameters and parameters of working process using in-cylinder events recording and analyzing. Mathematical simulation allows the assessment of the impact of structural and regulatory parameters on engine energy efficiency and environmental characteristics for a broad range of parameter values, which otherwise would be very expensive, time-consuming, and sometimes impossible to evaluate experimentally. The experiments in this approach are of small scale and mainly used for calibration and verification of simulation models. Other approaches use mathematical modelling and sophisticated experimental procedures simultaneously for the identification of unknown or partially known simulation model parameters. On the other hand, some of the papers published in this issue were of purely experimental character and mathematical methods were mainly used for processing the experimental results.