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Fuel direct injection represents one of the key turning points in the developmentof the Diesel engines. The appeal of this solution has been growing thanksto the parallel advancement in the technology of the injection hardware and inthe knowledge of the physics involved in the spray formation and combustion.In the present thesis, the effect of partial needle lift and injection rate shapinghas been investigated experimentally using a multi-orifice Diesel injector.Injection rate shaping is one of the most attractive alternatives to multipleinjection strategies but its implementation has been for long time impeded bytechnological limitations. A novel direct-acting injector prototype made it possibleto carry out the present research: this injector features a mechanical couplingbetween the nozzle needle and the piezo-stack actuator, allowing a fully flexiblecontrol on the nozzle needle movement and enabling partial needle lift as wellas the implementation of alternative injection rate shapes typologies. Differentoptical diagnostics were applied to study the spray development and combustionin a novel continuous flow test chamber that allows an accurate control on awide range of thermodynamic conditions (up to 1000K and 15MPa). In addition,hydraulic characterization tests were carried out to analyze the fuel flow throughthe injector nozzle.Partial needle lift has been found to affect the injection event, reducing themass flow rate (as expected) but also causing a reduction in the effective orificearea and an increase on the spreading angle. Moreover, at this condition, higherhole-to-hole dispersion and flow instabilities were detected. Needle vibrationscaused by the needle interactions with fuel flow and by the onset of cavitation inthe needle seat are likely the causes of this behavior.Injection rate shaping has a substantial impact on the premixed phase of thecombustion and on the location where the ignition takes place. Furthermore, theresults proved that the modifications in the internal flow caused by the partialneedle lift are reflected on the ignition timing. On the other hand, the analysis ofthe experimental data through a 1D spray model revealed that an increasing massflow rate (e.g. ramp or boot injection rate profiles) causes an increase in the fuelairequivalence ratio at the lift-off length and a consequent higher soot formationduring the diffusive phase of the combustion. Finally, the wide range of boundaryconditions tested in all the experiments served to draw general conclusions aboutthe physics involved in the injection/combustion event and, in some cases, toobtain statistical correlations.

PARTIAL NEEDLE LIFT AND INJECTION RATE SHAPE EFFECT ON THE FORMATION AND COMBUSTION OF THE DIESEL SPRAY