The aim of this thesis project is to study the combustion of ICE engines fuelled with natural gas, NG, and mixtures of natural gas with a percentage of hydrogen, HCNG. The work is carried out to support the European research project called GasOn whose target is to realize a sustainable and more efficient mobility in Europe, improving the energy efficiency of the powertrain. The use of low carbon alternative fuels like HCNG, will play in fact a fundamental role on the de-carbonisation process and it is beneficial for the reduction of pollutant emissions. A powerful tool able to study thecombustion process in engines is the Computational Fluid Dynamics, CFD, analysis: this approach is based on the theory of the fluid mechanics and it allows to solve problems related to fluid flows.