Impact of Mixture Inhomogeneity and Ignition Location on Early Flame Kernel Evolution in a Direct-Injection Hydrogen-Fueled Heavy-Duty Optical Engine

An optically accessible hydrogen-fueled, heavy-duty engine was used to investigate the impact of mixture formation on the early flame kernel propagation and the resulting combustion cyclic variability. Direct injection from a centrally mounted medium-pressure outward-opening hollow-cone injector created a fuel- air mixture with a global equivalence ratio of 0.33. The engine was operated at 1200 RPM with dry air at an intake pressure and temperature of 1.0 bar and 305 K, respectively. The charge was ignited at three different locations using focused-laser ignition to allow for undisturbed flame evolution, and the fuel injection timing and injection pressure were varied to influence the mixture inhomogeneity. High-speed OH* chemiluminescence imaging through a piston-crown window allowed for tracking the flame evolution while fluorescence imaging of anisole seeded into the hydrogen fuel provided two-dimensional information on the mixture distribution around the ignition location just before ignition. The results reveal that primarily the in-cylinder bulk-flow motion in conjunction with injection-induced flow influence the early flame kernel evolution. Despite the ultra-lean conditions, combustion was fast and fairly stable under most operating conditions, but the turbulence and inhomogeneity induced by fuel injection during the compression stroke significantly accelerated combustion compared to early injection during the intake stroke. Operating points with highly variable fuel/air mixture distribution near the ignition location exhibited increased cyclic variability with a few misfires.