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In an optically accessible single-cylinder engine fueled with hydrogen, acetone planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) are used to evaluate in-cylinder mixture formation. The experiments include measurements for engine operation with hydrogen injection in-cylinder either prior to or after intake valve closure (IVC). Pre-IVC injection is used to produce a near-homogeneous mixture for PLIF calibration experiments and to establish a baseline comparison for post-IVC injection. Calibration experiments and a temperature correction allow conversion of the acetone fluorescence signal to equivalence ratio. For post-IVC injection with start of injection (SOI) coincident with IVC, PLIF results are similar to pre-IVC injection. With retard of SOI from IVC, mixture inhomogeneities increase monotonically, with high hydrogen concentration spatially located near the injector and within a smaller volume.

The in-cylinder charge motion during the compression stroke of an optically accessible engine equipped with direct injection of hydrogen fuel is measured via particle image velocimetry (PIV). The evolution of the mean flow field and the tumble ratio are examined with and without injection, each with the unmodified 4-valve pent-roof engine head and with the intake ports modified to yield higher tumble. The measurements in the vertical symmetry plane of the cylinder show that intake modification produces the desired drastic increase in tumble flow, changing the tumble ratio at BDC from 0.22 to 0.70. Either intake-induced flow is completely disrupted by the high-pressure hydrogen injection from an angled, centrally located single-hole nozzle. The injection event leads to sudden reversal of the tumble. Hence the tumble ratio is negative after injection. However, the two intake configurations still differ in tumble ratio by about the same magnitude as before injection.