Hydrogen is a carbon-free alternative to fossil fuels that can be used in internal combustion engines allowing the transfer of know-how gained over the last 100 years of internal combustion engine development. Mass market adoption of these powertrains depend on making them zero-impact vehicles. This means restricting their exhaust emissions even though they are, in most cases, lower than the gasoline counterparts. With this in mind, experiments were carried out in an optically accessible hydrogen-fuelled engine to understand flame propagation through the piston top and ring lands, and to understand oil combustion in these regions. The results were then compared to similar phenomena with gasoline operation in the same engine. A correlation between the observed optical phenomena in the piston lands and the exhaust measurement was also derived which lays the path for improvement of hydrogen internal combustion engines towards zero-impact-emission powertrains. Moreover, indications for the cause of abnormal combustion phenomena could be observed.