Effect of a cross-flow on spray impingement with port fuel injection systems for HCCI engines

The use of multipoint injection systems for HCCI engines requires a mixture composition precisely controlled in order to obtain the optimum auto-ignition timing. The most favorable injection timing to achieve it is when the intake valve is open, in which case the fuel spray interacts with the air flow and impacts onto interposed surfaces where it may form a liquid film, especially at engine cold-start, and eventually generate smaller droplets by mechanisms of secondary atomization. An accurate description of the physics of spray-wall impact under cross flow conditions and of the effects of the latter on secondary atomization are key issues to devise appropriate strategies to optimize the injection system and to control mixture preparation. This is the aim of the work reported here. The experiments consider overlapping Mie and Shadowgraph visualization techniques and using a phase-Doppler interferometer to visualize and quantify the effects of a cross-flow on spray impact and secondary droplets. Furthermore, the experiments are conducted for well-defined boundary conditions, thus provide results useful for the development spray/wall interaction models. Analysis show that: the cross-flow decelerates the axial velocity of impinging droplets, thus decreasing the energy available at impact; because drag is more efficient on droplets more prone to adhere to the wall, the net effect is the formation of thinner films; this, in turn, enhances the generation of secondary drops which are then dragged away from re-impacting onto the wall.