Direct injection strategies for H2 internal combustion engines under ultra-lean conditions: a numerical analysis

Direct injection for hydrogen ICEs is a key technology to reach high volumetric efficiency while reducing the risk of abnormal combustion. Here, numerical simulations have been run to delve into the influence of the start of injection and injection pressure on the mixture formation process. Moreover, non-circular nozzle sections have been considered to increase mixing and multi-injection strategies to stratify the charge to speed up combustion under ultra-lean conditions (equivalence ratio of 0.25), trying to limit NOx emissions. Indeed, the combustion of a homogeneous ultra-lean mixture is significantly slow; stratification allows to increase the peak pressure up to 63%, but NOx emissions increase from a negligible amount to around 270 ppm. The proposed multi-injection approach allowed an increase of the peak pressure up to 51% while producing only 95 ppm of NOx. Therefore, this approach is a promising solution to optimize the heat release and emissions under ultra-lean conditions.