In an optically accessible single-cylinder engine fuelled with hydrogen, OH* chemiluminescence imaging is used to measure flame propagation characteristics, cycle-to-cycle variability, and to evaluate global trends related to 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 in-cylinder and to establish a baseline comparison for post-IVC injection. In order to assess the effects of injection pressure on mixture distribution and combustion, two injection pressures of 2.5 and 10 MPa are used for post-IVC injection. High flame propagation speeds and low cycle-to-cycle variability are observed for pre-IVC injection. For post-IVC injection with start of injection (SOI) coincident with IVC, results are similar to pre-IVC injection. With retard of SOI from IVC, mixture inhomogeneities increase monotonically for both injection pressures, but the spatial and temporal development of the mixture distribution differs significantly. Next, using the pre-IVC injection data sets, a functional relationship between the OH* chemiluminescence intensity, cylinder pressure, and equivalence ratio is determined. This functional relationship is then used to extract a semi-quantitative measure of the maximum line-of-sight averaged fuel-air ratio from the OH* chemiluminescence images acquired for post-IVC injection.
