Diluted combustion CFD analysis in a SI engine running on hydrogen-NG blends

In the modern scenario of pollutant emission regulation, the next EU fleet-wide average emission target to be reached before 2021 for new cars, is 95 g CO2/km. Because of high hydrogen to carbon ratio, methane is capable to reduce significantly CO2 emissions, with respect to a gasoline or diesel engine of the same power output. Moreover, methane is potential fuel for large scale supply and it is characterized by high octane rating which can help in increasing engine efficiency by operating with higher compression ratios. The present work has been carried out in a research team in DENERG laboratory at Politecnico di Torino. The aim of the thesis is the development of a model for 3D CFD simulations of an internal combustion engine fueled with natural gas, using CONVERGE CFD software. Once the model has been calibrated and validated with experimental data, for pure methane and two different mixture of hydrogen and natural gas (HCNG blends), it is used to predict the behaviour of the engine in case of dilution with exhaust gas (EGR) in different percentage. The post-processing analysis is focused on the combustion process and in particular on the effect of hydrogen addition, exhaust gas recirculation (EGR) and turbulence intensity on the stability of combustion and on the flame propagation in the chamber. One of the drawbacks in the use of natural gas is the slow burning velocity. This parameter can be however increased by adding hydrogen, thus reducing the combustion duration and improving the flame development. Another powerful way to increase the burning speed is the increase of turbulence inside the chamber: this causes higher wrinkling of the flame and faster exchange of heat and mixing of species. In this way the overall combustion process results to be substantially improved.