CFD analysis of combustion in CNG/HCNG engines

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 the combustion 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. Nowadays, there are several software in the market able to run CFD simulations; the commercial code used in this work is CONVERGE, provided by Convergent Science GmbH (Austria) company where this thesis has been carried out. Furthermore, thanks to the collaboration with Politecnico di Torino and in particular with the DENERG laboratory, the real performances of the FIAT FIRE 1.4 16V Turbo engine have been compared with the numerical results obtained from the simulations. The study is focused on two different fuels, each containing distinct percentages of hydrogen: 15% and 25% of H2. The validation of the model is based on the main characteristic parameters of the combustion, such as pressure, dissipation of energy inside the cylinder and finally pollutant emissions. The combustion process has been simulated through the SAGE detailed chemistry model; furthermore, different reaction mechanisms have been used in order to find the numerical model which simulates the real behaviour of the engine as well as possible. The thesis can be split into three main parts. The first part introduces the numerical modelling, with a brief description of the fluid governing equations useful for the CFD analysis, the discretization approaches and the base concepts of the CONVERGE software. The second part is focused on the main characteristics of the engine and lists the experimental data obtained in laboratory. In addition, it describes the case setup of the engine itself on the CONVERGE interface, showing for example the CAD geometry, the boundary conditions and the combustion and turbulence models used. Finally, the third part compares the numerical results with the experimental data, showing all the changes made to the case setup in order to reach the best possible results.