Most of the physicochemical mechanisms associated to the combustion process in diesel engines depend mainly on the characteristics of the fuel injection process. Besides the operating conditions (pressure and injection law, in-cylinder air density), the size and geometry of the injection nozzle holes do have a significant effect on this process. Although it is not the only one, the main consequence of the geometry of the holes on the injection process is the presence or absence of the cavitation phenomenon. Many authors have studied this phenomenon in diesel injectors.

The present work starts with a literature review, where it is possible to conclude that few studies have been focused on analyzing the consequences of the presence of cavitation on different aspects of the injection, evaporation and soot formation mechanisms. Some of these few studies have shown that, besides of the mass flow rate choking (which is the most observed effect), the presence of cavitation leads to a significant increase in effective injection velocity and an acceleration of the mixing and combustion processes. In order to confirm these experimental results, the main objective of the current thesis is defined as to deeper understand the influence of cavitation in diesel injection nozzles on the injection and soot formation processes.

To achieve the above objective, the present study has been divided in three parts, in which the following items have been used: on the one hand, two three-hole nozzles, one with a high level of cavitation (cylindrical nozzle) and another one in which this phenomenon is inhibited (conical nozzle), and, on the other hand, both experimental and theoretical tools. In the first part, the effect of cavitation on the effective injection velocity has been studied first, and afterwards the reason why cavitation provokes an increase in this velocity has been investigated. The increase of the effective injection velocity when cavitation appears has been confirmed and, in addition, it was found that this increase is due to a change in the velocity profile inside the injection hole as a consequence of the decrease of fluid viscosity when cavitation appears.

In the second part, the effect of cavitation on the mixing process, and more precisely on the spray cone angle, has been studied. This process has been characterized in two different scenarios: firstly, from the information obtained from liquid length images (in conditions close to the real ones, i.e. evaporative but non-reacting spray), where two spray angles were analyzed, one obtained from the functional dependency of the liquid length and the other obtained by direct measurement from the images of liquid length. And, secondly, by means of the heat release fraction information (in real conditions, i.e. evaporative and reacting spray). In both scenarios the increase in spray cone angle when cavitation appears has been confirmed.

Finally, in the third part, the influence of cavitation on the lift-off length and on the soot formation process has been studied, where this last process has been characterized in two different scenarios: by measuring the soot radiation and by measuring the final soot emissions. Concerning the lift-off length, this parameter is modified by cavitation as the latter produces an increase in effective injection velocity (which would increase this length) and a reduction in effective diameter (which would decrease this length). Apart from these expected trends, the results show that cavitation provokes an additional increase in this length, probably due to the increase in the level of turbulence of the flow, which leads to a stabilization of the lift-off further downstream. And concerning the soot formation process, in both scenarios it was observed that the cylindrical nozzle produces less soot compared to the corresponding conical nozzle with an equivalent permeability. This result is closely related to the previous one, as the increase in lift-off length provoked by cavitation leads to a reduction in the relative fuel/air equivalence ratio at this length, which would explain the decrease in soot formation rate. Based on the results obtained in the present work, the cylindrical nozzle under cavitating conditions may offer a potential to reduce soot emissions.