Optimization of piston bowl geometry in terms of fuelconsumption and emissions using computational fluiddynamics in a diesel engine

In this thesis, the piston of a Diesel engine has been optimized using Computational Fluid Dynamics (CFD) with respect to Specific Fuel Consumption. With the help of one-dimensional gas exchange analysis, the engine performance characteristic was examined. The AVL BOOST program was used for one dimensional gas exchange analysis and to identify the engine performance characteristics. Swirl analysis of the engine with double suction manifold was studied. The commercial CFD software STAR-CD was used for engine swirl investigations. First, a detailed coarse, medium and fine mesh study were carried out for the swirl computations. It was observed that the results of fine and medium mesh were quite similar such that the rest of the CFD simulations could be carried out on the medium computational mesh. Standard k-e, Realizable k-e and RNG k-e turbulence models were used to see the effects of turbulence modelling on the engine swirl number. A sub module of STAR-CD, ES- ICE is used for the engine moving valve and piston events where cell-layer addition and deletion is employed in the cylinder domain while piston moves down and up respectively. Reacting Computational Fluid Dynamics (RCFD) simulations have been vii performed for a 45 degree sector geometry that corresponds to one fuel injector hole making use of the periodicity boundary condition. Computations are carried out for the duration of 200° crank angle (CA) between 600° – 800° CA while the intake and exhaust valves are closed. ECFM-3Z, Extended Coherent Flame Model - 3 Zone is used as combustion model. A lagrangian approach is used for two phase modelling of the liquid Diesel fuel. Reitz and Diwakar primary break-up modelling coupled with either Bag Break-Up or Stripping Break-Up modelling are used to simulate the spray formation and atomization. Spatial and temporal discretization and turbulence modelling effects have been carefully studied. The CFD predictions of the in-cylinder pressure and Apparent Heat Release are compared with the measurement data to validate the fidelity of the results. The reactive CFD predictions using the medium computational mesh with the temporal time step that corresponds to 0.1° Crank Angle shows a discrepancy of 2.3% compared to the measured data. After the validation of the predictions the geometrical parameters of the piston bowl were determined for optimization study. The SHERPA algorithm of the HEEDs program was used to optimize the chosen 9 geometrical parameters with respect to Specific Fuel Consumption (SFC). As a result of the analysis of 92 different geometries, an improvement of 5.8% in terms of power and 5.5% in terms of specific fuel consumption was achieved. However, there was one penalty of the optimized geometry in terms of NOx emission that the optimized newly designed piston bowl geometry yields a 15% increase in NOx emissions compared to the current baseline geometry.