Visual Analyses of End of Injection Liquid Structures and the Behaviour of Nozzle Surface-Bound Fuel in a Direct Injection Diesel Engine 2019-01-0059

For efficiency, the majority of modern diesel engines implement multiple injection strategies, increasing the frequency of transient injection phases and thus, end of injection (EOI) events. Recent advances in diagnostic techniques have identified several EOI phenomena pertinent to nozzle surface wetting as a precursor for deposit formation and a potential contributor towards pollutant emissions. To investigate the underlying processes, highspeed optical measurements at the microscopic scale were performed inside a motored diesel engine under low load/idling conditions. Visualisation of the injector nozzle surface and near nozzle region permitted an indepth analysis of the post-injection phenomena and the behaviour of fuel films on the nozzle surface when the engine is not fired. Inspection of the high-speed video data enabled an interpretation of the fluid dynamics leading to surface wetting, elucidating the mechanisms of deposition and spreading. As the needle re-seated, the abrupt pressure drop inhibited atomisation. Large, slow moving, liquid structures were released into the cylinder with the capability of impinging on nearby surfaces, creating localised fuel rich regions, or escaping through the exhaust and contributing towards un-burnt hydrocarbon emissions. Large ligaments remained attached to the nozzle, with some fluid subsequently breaking away while the remaining fuel adhering the nozzle retracted back causing surface wetting. The EOI event was succeeded by further surface wetting due to the expansion of orifice-trapped gas dislodging nozzle-residing fuel that then overspilled onto the external surface. The drop in in-cylinder pressure elicited bubbling within the surface-bound fuel, further increasing the films spreading rate. The resulting bubble agglomerations collapsed in large chain reactions, projecting more fuel into the cylinder. Finally, as the intake valves closed, high velocity intake air was diverted towards the nozzle removing the remaining surface-bound fuel. As a result, a large volume of fuel was released into the combustion chamber after the EOI causing deposits on nearby surfaces or getting released through the exhaust where it would contribute towards un-burnt hydrocarbon emissions. It is likely that the anticipated increase in in-cylinder pressure and temperature if the engine was fired would either reduce the time-scale of these event or completely inhibit them. However, understanding the behaviour of the surface-bound fuel within this environment will aid designs that control surface wetting, thus inhibiting nozzle coking with the capacity to control internal deposits.