Development of a 3D-CFD Model for the Analysis of Combustion and Emissions in a Light-Duty Diesel Engine

In order to reduce emissions of carbon dioxide from engines, a shift to carbon-neutral fuels is one of the promising alternatives. Hydrotreated vegetable oils (HVOs) are very high-quality bio-based diesel fuels, which do not have negative effects on engines, exhaust aftertreatment devices, or exhaust emissions. A research activity has been started at the Energy Department of Politecnico di Torino to investigate effects of using HVO instead of a conventional diesel fuel in a production engine. As a first step, a light duty commercial compression-ignition engine fueled with the conventional diesel fuel has been tested. In the present thesis, as the next step of the activity, a validation and sensitivity analysis of 3D-CFD (three-dimensional computational fluid dynamics) model is presented. It enables the assessment of the impact of mesh size, swirl number, and turbulent Schmidt number variations on simulated diesel combustion and pollutant emissions. The 3D-CFD analysis is carried out using the CONVERGE CFD software on a 45-degree sector of the symmetrical combustion chamber from IVC (intake valve closing) to EVO (exhaust valve opening). Engine performance is simulated at 2000 rev/min and two different loads: full load (18 bar BMEP) and part load (9 bar BMEP). The model is validated against the experimental data in terms of in-cylinder pressure, heat release rate, NOx and soot emissions. The same parameters are used to quantify the influence of the mesh size on the accuracy of numerical results, taking into account the computational cost involved when improving mesh refinement. As the initial flow field within the cylinder at IVC in this work is defined using data available from experiments and GT-POWER 1D simulations, the sensitivity of diesel combustion simulations on the initial swirl number is investigated. Also, additional sensitivity analysis is performed by changing the turbulent Schmidt number, which is an important parameter influencing the transport of species during 3D-CFD simulation. Results of the sensitivity analysis yield useful indications on definition of initial conditions for 3D-CFD simulations, as well as on further model calibration possibilities. In the first part of this thesis, a theoretical background on flow and combustion in diesel engines, CFD fundamentals, and the CONVERGE CFD software is provided. The second part deals with preparation and calibration of the CFD model. Finally, in the third part, results of the sensitivity analysis are provided and explained. In the future, the following step of the research activity will include using HVO instead of the conventional diesel in the CFD model developed during the present thesis. The aim will be to investigate the influence of HVO on the combustion process and formation of pollutants.