Computational Fluid Dynamics investigation on Ducted Fuel Injection operation with Multi-Injection strategies.

Ducted Fuel Injection (DFI) is a concept of growing interest to mitigate soot formation in the diesel combustion process and, simultaneously, break the usual trade-off between soot and Nitrogen Oxides (NOx) emissions. Particularly, the concept is to inject a fuel spray down the axis of a small duct, positioned within the combustion chamber some distance downstream of the injector orifice exit, so that the mixing between air and fuel is increased; as a consequence, a lower equivalence ratio as well as lower engine-out soot emissions are expected. However, the research studies conducted so far have been mainly performed considering only a single-injection event, while state-of-the art diesel engines typically feature highly optimized Multi-Injection (MI) strategies to address emissions, combustion noise and engine performance issues. Therefore, it still remains nowadays an open point, in scientific literature, the synergistic exploitation of both MI strategy and DFI technology. Particularly, this aspect is of paramount importance to achieve cleaner and more efficient combustion engines. In this framework, the present master thesis work aims to analyse the impact of different MI strategies on the DFI operation in constant-volume conditions. For this purpose, a spray model has been developed in the 3D Computational Fluid Dynamics (CFD) environment, extensively validated against experimental data for single-injection strategies, considering both free spray (i.e., a fuel spray that is not surrounded by a duct) and DFI configurations, thus ensuring the reliability of the numerical simulations. After that, predictive non-reacting and combustion simulations have been performed to investigate the effect of MI strategies with DFI. Concerning the injection profiles, split injections with several electric dwell times and shares of injected mass have been tested. The equivalence ratio, the turbulence intensity, the temperature and the soot formation and oxidation processes have been analysed and compared for each case. From an overall point of view, the simulations results have confirmed the great soot mitigation potential enabled by the duct adoption, even when MI strategies are concerned. Indeed, considering the non-reacting simulations, the interaction among different injection events did not drastically modified the narrower equivalence ratio distribution characteristic of the DFI case, leading to lower values especially in the area where free-spray cases are characterized by the presence of a rich core. This characteristic led to a reduction of the soot emissions during the correspondent combustion simulations. In conclusion, this work suggests that the DFI can be combined with MI strategies, representing a step forward in the development of this technology towards its employment in low-soot series-production diesel engines.