This Special Issue on "New Trends on the Combustion Processes in Spark Ignition Engines" contains nine papers on new developments on Internal Combustion (IC) engines aiming to enhance their efficiency, leading to the reduction of fossil CO2 and other gaseous pollutants. It is divided into two parts. In the initial part, the focus in on fuels, with four papers discussing the use of biofuels and other alternative fuels that can be used in different types of IC Engines. Additionally, conventional fuels are tested in order to evaluate their optimal use in new downsizing high-boost engines. A revision paper on alternative fuels is also included. The second part involves the study and improvement of engine combustion diagnostics as well as the presentation of an alternative type of propulsion system.