**6. Conclusion**

The application of CFD techniques to the study and optimisation of the combustion process in a GDI engine is presented.

A first part of the work is devoted to the numerical multidimensional modelling of the dynamics of sprays issuing from new-generation multi-hole injectors for GDI applications.

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Three injectors are preliminary experimentally investigated with the aim of building a comprehensive database for the assessment of a proper spray model. The injectors are all characterised, under various injection strategies, in terms of both measurement of the instantaneous mass flow rate and visualization of the fuel dispersion in an optically accessible vessel. The developed spray model exploits a log-normal distribution of the initial droplets size of given expected value and variance. The expected value is theoretically determined as a function of the experimental injection velocity and backpressure, whereas the variance is properly tuned, together with a constant regulating the adopted droplets break-up model. Tuning of the constants is realised by assessing an automatic novel procedure within an optimisation software. The model portability with respect to the injection pressure and the kind of injector is demonstrated.

A 3D CFD model able to define the main guidelines for the management of the mixture formation process in a high performance GDI engine is then assessed. Simulation of the whole four-stroke engine cycle is effected by considering gasoline adduction during intake through one of the tested injectors. Boundary and initial conditions for the 3D model are defined, as a function of time, on the ground of a 1D simulation of the whole propulsion system.

The mixture formation process under high-speed high-load working conditions is shown to be strongly affected by the choice of the angle of inclination of the injector axis w.r.t. the cylinder axis. For lower angles, the gasoline droplets remains confined in the vicinity of the walls or quickly reach the surface of the piston, where they remain for a long time, especially during compression due to the motion of the piston itself. The choice of the SOI is made in order to fully exploit the turbulent motion of the entering air. The injection pressure must not be increased so much in order to limit the NO formation.

A moderate speed, moderate load condition is also studied into detail, where the air-to-fuel ratio is maintained lean. Both single and double injection events are considered. Optimal choice of both the start of the single injection strategy and the time of spark advance is realized by means of the Simplex algorithm, in order to minimize the fuel consumption. The same criterion is adopted in the numerical study of mixture formation and combustion consequent a double injection. The start of the first pulse and the dwell time between two successive pulses, both characterized by the same gasoline mass, are searched, together with the best time of SI. The optimal solution is shown to reduce the fuel consumption with respect to the case injection is realized in one shot, thus confirming the possibility to resort to split injections to improve the quality of the charge stratification under lean operation.

The assessed procedure, where a properly developed 3D engine model is coupled with an optimization tool, is proven to be a valuable tool in the phase of engine design.
