This issue is devoted to a quite new scientific and engineering topic - up to now not treated in the papers published in our journal Thermal Science ? investigation of thermal processes in internal combustion engines. This decision of the Editorial board was initiated by a quite large number of papers submitted for publication dealing with those problems, and particularly oriented towards the use of biofuels. In order to encounter in one issue different connected problems and processes in internal combustion engines, as one of the very important thermal machine, Editorial board asked distinguished experts Prof. Dragoslava Stojiljković and Prof. Stojan Petrović, to be Guest editors, and to make a more wide choice of problems and processes and to persuade researchers from different countries dealing with those topics to present their results in this issue.