The impact of fuel drops on the walls of combustion chambers is unavoidable in direct-injection automotive engines. These drop-solid interactions can lead to splashing of the lubrication oil, its dilution or removal, which can damage the piston or the liner from dewetting. This can also cause irregular and inferior combustion or soot formation. Understanding the drop-splashing dynamics is therefore important, especially as modern IC engines are being down-sized to achieve higher thermal efficiency. Typical cylinders of IC engines contain metal liners on their walls, which have fine azimuthal grooves to support the lubricating oil as the piston moves inside the cylinder. In this thesis we study how these grooves affect the deposition or splashing of impacting diesel drops, while the solid surface is kept dry without the lubricating oil. For these experiments we use sections of actual cylinder liners and apply high-speed video imaging to capture the details of the drop impacts.
The first set of experiments used normal impacts on horizontal substrates. These experiments include a range of drop sizes and impact velocities, to identify impact conditions in Reynolds and Weber number space where the transition from deposition to splashing occurs. We also study the maximum radial spreading factor of the impact lamella, finding about 8% larger spreading along the grooves than perpendicular to them.
In the second set of experiments we look at the impact on inclined substrates, where the inclination angle is between 30o–60o. This produces strong asymmetry in the maximum spreading, with the tangential velocity governing the maximum radial motion.
The inclined impacts change the splashing threshold, requiring larger impact velocities for splashing. The splashing threshold deviates quantitatively from earlier theories, but shows the same qualitative trends. Furthermore, a new splashing mechanism is observed, where the impact forms a prominent ejecta crown from the downstream edge. This crown ruptures first from the grooves at the sides and subsequently the capillarity detaches the downstream levitated liquid sheet from the substrate generating a myriad of splashed droplets.
Preliminary observations with impacts on wet substrates show much stronger crown-formation from the lubricating oil film, with potential for dewetting.