Spray-wall interaction and its effects on partially premixed combustion (PPC) at low engine load

Partially premixed combustion (PPC) often uses the early fuel-injection strategy that could result in the spray-wall interaction involved with piston top-land crevice. This interaction may produce a great impact on the 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 and spatial UHC distribution of PPC at low engine load using multiple optical diagnostic techniques. We observed the fuel-trapping effect of the piston top-land crevice and the low- and high-temperature heat release of the trapped fuel in the piston crevice by planar laser-induced fluorescence. We further analyzed the distribution characteristics of UHC under two different spray-wall interaction cases. Results indicate that the main UHC formed in PPC at low load arises from the central part of the cylinder above the piston bowl and close to the injector nozzle, where the overall equivalent ratio is low and the injector dribbling is an important source of UHC. The fuel trapped in the piston crevice under early fuel injection timing is not necessarily an important source of UHC emission when the local equivalence ratio of the fuel-air mixture is relatively high.