Mixture formation and combustion characteristics in a heavy-duty hydrogen DI Engine: Impact of tumble and injector configuration

This study investigates improving in-cylinder fuel mixing and combustion stability in a hydrogen-fueled direct-injected heavy-duty optical engine with flat-deck cylinder head by changing the injector outflow geometry and using tumble plates to alter in-cylinder flow. The medium-pressure outward-opening hydrogen injector was mounted in-between the intake valves inclined 41 degrees from vertical, and the injector outflow boundary was defined by the injector bore in cylinder head and the injector protrusion, which was varied. Anisole-tracer planar laser-induced fluorescence was utilized to obtain quantitative images of the equivalence ratio field at spark timing while high-speed OH* chemiluminescence imaging was used to characterize the flame evolution after ignition. The test matrix included two injector outflow boundaries defined by injector protrusion. For each of these, three levels of tumble were tested while varying the start of injection timing. This provided insight into the ability of tumble to offset deteriorated mixture quality and cycle-to-cycle variability as injection timing was delayed. Variation of the injector protrusion resulted in significant differences in mixture formation, which were explained by the near field jet visualization during injection. Images revealed that when the injector is recessed into the cylinder head, the cylindrical injector bore reshapes the hollow-cone jet into a single jet aligned with the bore. In the protruded injector configuration where the outflow from the injector was not bound by the injector bore, two distinct plumes were detectable in near-field, one of which penetrated downward along the cylinder liner and counteracted tumble motion. Higher tumble with the retracted injector improved mixing and supported efficient combustion with low cycle-to-cycle variability. The protruded injector configuration at late injection timings resulted in a highly stratified mixture with low fuel concentration near the spark plug, resulting in poor combustion stability. Flame centroid extraction from high-speed combustion imaging further revealed that the tumble motion in the tested engine configuration is not sustained into the late compression stroke, which reduced the expected benefits of tumble. These findings highlight the complexity of optimizing hydrogen combustion engines when improving thermal efficiency and minimizing NOx are primary objectives.