Development of a LIF-Imaging System for Simultaneous High-Speed Visualization of Liquid Fuel and Oil Films in an Optically Accessible DISI Engine

Downsizing and direct injection in modern DISI engines can lead to fuel impinging on the cylinder walls. The interaction of liquid fuel and engine oil due to fuel impinging on the cylinder wall causes problems in both lubrication and combustion. To analyze this issue with temporal and spatial resolution, we developed a laser-induced fluorescence (LIF) system for simultaneous kHz-rate imaging of fuel and oil films on the cylinder wall. Engine oil was doped with traces of the laser dye pyrromethene 567, which fluoresces red after excitation by 532 nm laser radiation. Simultaneously, the liquid fuel was visualized by UV fluorescence of an aromatic “tracer” in a non-fluorescent surrogate fuel excited at 266 nm. Two combinations of fuel and tracer were investigated, iso-octane and toluene as well as a multi-component surrogate and anisole. The fluorescence from oil and fuel was spectrally separated and detected by two cameras. Both the laser and the cameras were capable of kHz repetition rates. Preliminary studies in a thin-film cuvette investigated the optical properties of tracer/fuel-mixtures and mixtures with a first-fill engine oil and a low-additive surrogate engine oil. In particular, cross-talk by the fuel tracer in the oil channel was low, but signal from oil fluorescence in the fuel channel was significant. Experiments were performed in a research engine with optical access along the entire stroke of the cylinder in fired and motored engine operation. Besides fuel wall wetting, fuel transport across the piston ring pack and the impact on the lubricant conditions in the piston group can be seen in the images.