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The current paper describes a methodology for combustion bowl assessment for diesel engines. The methodology is based on the application of Computational Fluid Dynamics (CFD) following a Design of Experiments (DoE) procedure. In this work the 3D CFD simulation was performed by the commercial CFD code AVL-FIRE for different combustion bowls from intake valve closing (IVC) to exhaust valve opening (EVO). The initial conditions (at IVC) and boundary conditions were obtained from 1D simulation. Since the work was concentrated on the spray injection, mixing, combustion as well as bowl aerodynamics only a sector mesh was employed for the calculations. A DoE procedure was also used for this simulation work in order to minimize the number of simulation runs and at the same time maintaining the accuracy required assessing the influences of different bowl geometry, spray and intake air motion parameters.

On the basis of the newly-developed fractal combustion model, the engine-thermodynamic-cycle simulations were conducted with the 1D engine-cycle-simulation program AVL-BOOST for a passenger-car SI engine with a fully-variable valve train. Results of the simulations showed a good agreement with measurements for both full and part load at various engine speeds. On the basis of the thermodynamic model for the engine, the valve event optimization was carried out for both full and part load with a partial factorial DoE plan consisting of various valve event durations and timings. For each of the selected cases, an independent optimization for the ignition timing was performed to determine the minimum BSFC under a constraint on specified knock criteria. Satisfactory results for the valve event optimization were achieved.