The ever more demanding of fuel economy and emission standards require innovative control strategies for internal combustion engines (ICE) and new procedures for the use of alternative fuels as well as improved after-treatment systems. This development is intrinsically linked to improved understanding of in-cylinder processes, with complex dependencies between phenomena that are related to flow, thermal and chemical engineering. In this thesis, kernel formation and flame front propagation were investigated in an optically accessible Direct Injection Spark Ignition (DISI) engine. The optical analysis was made possible by a Bowditch design that features an extended piston combined with a quartz window mounted within the piston crown.