Partially premixed combustion (PPC) often adopts the early fuel-injection strategy that could result in spray-wall interaction involved with piston top-land crevice. This interaction may produce a significant impact on engine combustion and unburned hydrocarbons (UHC) emission, which is still not well understood. In this study, we investigated the detailed spray-wall interaction and its effects on the two-stage ignition, i.e. low- and high-temperature heat release (LTHR and HTHR), and the in-cylinder spatial UHC distribution of PPC in a full-view optical engine at low engine load. The PRF 70 fuel was used as the gasoline surrogate. The high-speed imaging of the natural flame luminosity was acquired to quantify the flame probability distribution. The qualitative fuel-tracer, formaldehyde, and UHC planar laser-induced fluorescence (PLIF) imaging techniques were employed to reveal the fuel, LTHR and UHC distribution characteristics, respectively. The LTHR, HTHR and UHC distribution formed by the fuel trapped in the piston top-land crevice were visualized by PLIF imaging techniques for the first time. The PLIF results indicate that the main UHC formed in the PPC engine comes from the central part of the cylinder close to the injector nozzle, where the overall equivalence ratio is low and the injector dribbling is an important source of UHC. The UHC formed in the piston crevice of the PPC engine depends on the local equivalence ratio of the fuel trapped in the crevice. When the overall equivalence ratio of the charge in the crevice is relatively high, the trapped fuel undergoes both LTHR and HTHR and produces negligible UHC. However, the UHC from the piston crevice becomes considerable when the fuel injection timing is too early so that an overly lean mixture is generated. Based on the above findings, three implications of the PPC operation at low engine load for low UHC emission and high engine efficiency are proposed.
