Influence of the Flow Field on Flame Propagation in a Hydrogen-Fueled Internal Combustion Engine

Flame propagation in an optically accessible hydrogen-fueled internal combustion engine was visualized by high-speed schlieren imaging. Two intake configurations were evaluated: low tumble with a tumble ratio of 0.22, corresponding to unmodified intake ports, and high tumble with a tumble ratio of 0.70, resulting from intake modification. For each intake configuration, fueling was either far upstream of the engine, with presumably no influence on the intake flow, or the fuel was injected directly early during the compression stroke from an angled single-hole injector, adding significant angular momentum to the in-cylinder flow. Crank-angle resolved schlieren imaging during combustion allowed deducing apparent flame location and propagation speed, which were then correlated with in-cylinder pressure measurements on a single-cycle basis. In a typical cycle, flame shape and convective displacement are strongly affected by the in-cylinder flow. For homogeneous fueling with low tumble, the flame is convected little, growing without significant wrinkling with a shape that is quite symmetric in the vertical plane. In contrast, in the other cases the flame is convected and stretched. Ensemble averaged results show that for fully homogeneous conditions the increase in tumble ratio from 0.22 to 0.70 results in increased flame growth and shorter combustion duration. For the stratified mixture, two regimes were observed: Early in the combustion, the flame grows faster for high intake-induced tumble, while during middle and late combustion low tumble yields a faster burn rate with an overall shortest combustion. On a single-cycle basis, early flame growth strongly correlates with the crank angle at which 5% of the fuel mass is burned. Convection is characterized by the displacement of the flame's projected area centroid, revealing that the multi-cycle centroid cloud spreads with time and that the cycles follow different paths corresponding to their flame speed: typically the slow cycles stay near the ignition point and at the top of the centroid cloud. For direct injection, the ensemble average centroid speeds are relatively high in the beginning and then slowly decrease. In contrast, with homogeneous fueling the centroids have nearly constant convective speed.