Experimental Assessment of a Prototype Hollow Cone Spray Diesel Injector by Means of Spray and Engine Characterization

The application of more efficient combustion concepts requires the advancement of fuel injection technologies. The injector nozzle is the most critical component of the whole injection system for its impact on the combustion process. It is characterized by the number of holes, diameter, internal shape, and opening angle. The reduction of the nozzle hole diameter seems the simplest way to promote the atomization process, but consequently, the number of holes should increase to keep constant the injected fuel mass. This logic has been applied to the development of a new generation injector. First, the tendency to increase the nozzle number and to reduce the diameter has led to the replacement of the nozzle with a circular plate. The vertical movement of the needle generates an annulus area for the fuel delivery on 360 degrees, so controlling the atomization as a function of the vertical plate position, capable of generating a hollow cone spray. Second, on the base of the obtained results, different configurations were introduced. These new configurations are characterized by a hybrid fuel injection concept, composed of preferential fuel jets, generated via a specific design of the internal nozzle geometry, and additional circle-shaped fuel injection, to use the air between the jets. An additional hollow cone nozzle injector was designed, characterized by a higher spray cone angle with respect to the first prototype. The experimental investigation of all new configurations has been performed on a combustion vessel, to characterize the spray evolution in a quiescent ambient in engine-like conditions by means of optical diagnostic techniques, and on a single-cylinder engine to assess the injectors performances by varying the main control parameters (electrical command, nozzle tip protrusion, test point, and calibration parameters). A more in-depth investigation of the injector characteristics in different conditions has also been performed thanks to the hydraulic characterization on the hollow cone nozzle prototype. On the base of optical and fluid dynamic diagnostics, a CFD code was also validated in evaporative and non-evaporative conditions. The optical analysis showed a high spray evaporation rate, and the aspect also emerged in the engine results. The hydraulic characterization by Bosch tube meter highlighted a high flow rate with small needle lift and reduced hydraulic delay, compared to conventional multi-hole. The numerical simulation allowed pointing out a rapid breakup process immediately for the HCN injector. In general, the latest versions of hybrid injectors have achieved quite similar performances, in terms of spray penetration, compared to the conventional multi-hole nozzle, while guaranteeing a different fuel distribution. Based on the engine results, the HCN multi-jets versions guarantee better performance in comparison to the HCN old version, but it has not yet reached a level as to be considered a valid alternative to MHN. The hybrid version of HCN, thanks to the increase of the number of jets, has the potential to improve the overall spray-combustion chamber interaction further and then the engine performance.