This issue of the journal Thermal Science is once again dedicated to the problems of internal combustion engines. A thermal process inside the engine dictates engine thermal efficiency and environmental level. The complexity of the process is caused by interlacing processes of thermodynamics, fluid mechanics, and chemical kinetics. Therefore theoretical and experimental research in this area is always current, causing that a large number of papers was in competition for the publication in this issue, and the most interesting are published, presenting the results of research in the area of engine exhaust emissions, engine simulation and modeling, and the use of alternative fuels. Engine exhaust emissions problem is still very important, because stringent regulations require continual research and application of new solutions to reduce pollutants from the engine. Research of NOx and particulate matter emissions is in the first place, especially having in mind new requirements for measurement of fine particles emission. The study of optimal working process has always attracted researchers. The complexity of work ing process makes this problem very difficult. Consideration of theoretical thermodynamic cycle gives the basic assumptions of efficiency and possible optimization of the process. The use of finite time thermodynamics to analyze theoretical cycles is very useful. However, the real working process can be simulated with more complex mathematical apparatus using CFD techniques. Studies of the application of different approaches in multi dimensional modeling of engine process are presented in this issue, as well as different approaches of exhaust manifold modeling (problems of junction, heat transfer, and aftertreatment) and simulation of fuel injection process. The problems of implementation of alternative fuels in order to replace fossil fuels, reduce emissions and im prove energy efficiency always attract the attention of researchers. The possibilities are to use hydrogen or its addition to conventional fuels, to use natural gas and to use ethanol or biodiesel (from different origins), as well as their blends. Also, one possibility to increase over all energy efficiency of transport is the renewal of existing vehicle fleet. All these problems are treated in this issue. We have chosen 25 papers, and unfortunately a number of papers on this subject will have to remain for the next specialis sue. Any way, we will like to take the opportunity to express our great gratitude to all authors, for their successful research work, as well to the reviewers, for their sharing time and knowledge to improve the quality of our journal. Also, many thanks to Prof. Dr. Simeon Oka, Editor-in-chief, who has enabled the publishing of this special issue.