Fuel tracer-based planar laser-induced fluorescence is used to investigate the vaporization and mixing behavior of pilot injections for variations in pilot mass of 1-4 mg, and for two injection pressures, two near-TDC ambient temperatures, and two swirl ratios. The fluorescent tracer employed, 1-methylnaphthalene, permits a mixture of the diesel primary reference fuels, n-hexadecane and heptamethylnonane, to be used as the base fuel.
With a near-TDC injection timing of −15°CA, pilot injection fuel is found to penetrate to the bowl rim wall for even the smallest injection quantity, where it rapidly forms fuel-lean mixture. With increased pilot mass, there is greater penetration and fuel-rich mixtures persist well beyond the expected pilot ignition delay period. Significant jet-to-jet variations in fuel distribution due to differences in the individual jet trajectories (included angle) are also observed. Increased injection pressure significantly increased the mixing rate, leading to leaner mixture distributions, and with lower ambient temperature modestly richer mixtures are found near the heads of the jets. Discrete droplets near the injector were unexpectedly found to be more common at the higher ambient temperature. Lastly, increased swirl displaces the fuel to locations lower in the bowl, while generating a greater amount of over-lean mixture in the upper-central region of the combustion chamber.
