Optical Investigation of Mixture Formation in a Hydrogen-Fueled Heavy-Duty Engine with Direct-Injection 2023-01-0240

Mixture formation in a hydrogen-fueled heavy-duty engine with direct injection and a nearly-quiescent top-hat combustion chamber was investigated using laser-induced fluorescence imaging, with 1,4-difluorobenzene serving as a fluorescent tracer seeded into hydrogen. The engine was motored at 1200 rpm, 1.0 bar intake pressure, and 335 K intake temperature. An outward opening medium-pressure hollow-cone injector was operated at two different injection pressures and five different injection timings from early injection during the intake stroke to late injection towards the end of compression stroke. Fuel fumigation upstream of the intake provided a well-mixed reference case for image calibration. This paper presents the evolution of in-cylinder equivalence ratio distribution evaluated during the injection event itself for the cylinder-axis plane and during the compression stroke at different positions of the light sheet within the swirl plane. During the injection event, the originally annular jet collapses onto the jet axis within 1°CA after jet emergence and within 10 mm downstream of the nozzle. Multiple shock cells are visible – their size decreases with decreasing pressure ratio. The results of the equivalence ratio distribution show high cyclic variability of mixing for all injection timings during the compression stroke, but only minor variability with early injection during the intake stroke. The ensemble-mean fuel distribution shows that fuel-rich zones shift from the intake side to the exhaust side of the combustion chamber as the injection is advanced. Probability density functions of global equivalence ratio and equivalence ratio at potential spark locations suggest that retarded fuel injection might significantly increase NO emissions and the cyclic variability of early flame kernel development.