Visual analyses of end of injection liquid structures and the behaviour of nozzle surface-bound fuel in a direct injection diesel engine

Multiple injection strategies are implemented in the majority of modern diesel engines, 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 potentially contribution to unburnt hydrocarbon emissions. To investigate the underlying processes, high-speed optical measurements at the microscopic scale were performed inside an idling diesel engine. Visualisation of the injector nozzle surface and near nozzle region permitted an in-depth analysis of the post-injection phenomena and the behaviour of fuel films on the nozzle surface. 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-seats, the abrupt pressure drop inhibited atomisation, with large, slow moving, liquid structures released into the cylinder with the capability of impinging on nearby surfaces, creating localised fuel rich regions, or escaping through the exhaust, contributing towards un-burnt hydrocarbon emissions. Large ligaments remained attached to the nozzle, subsequently splitting and retracting back onto the surface. The inertia of the fuel leaving the orifices is known to induce rarefaction within the nozzle, equalised through the ingestion of in-cylinder gas and surface-bound fuel. The EOI event was succeeded by further surface wetting due to the expansion of orifice-trapped gas dislodging nozzle-residing fuel and causing it to overspill onto the external surface. The drop in in-cylinder pressure elicited bubbling within the surface-bound fuel, further increasing the film’s spreading rate. The resulting film bubble agglomerations collapsed in large chain reactions, projecting more fuel into the cylinder. Large quantities of projected fuel were visibly drawn towards the exhaust valves, potentially contributing to un-burnt hydrocarbon emissions.