Numerical analysis of direct-injection and combustion inside a H2 engine

The direct injection of hydrogen (H2) inside internal combustion engines (ICEs) is gaining large research interest over the port-fuel injection strategy, because of several advantages as higher volumetric efficiencies, increased power output and reduced risks of abnormal combustion. However, the required high pressure ratios across the injector nozzle produce moderate-to-high under-expanded jets, characterized by complex flow structures. This poses a challenge for the numerical modelling of the mixture preparation by means of 3D computational fluid dynamic (CFD) approaches. In this work, a validated 3D-CFD methodology has been employed to simulate the closed-valve cycle of a direct injection H2 engine equipped with a centrally mounted hollow-cone injector and a non-axisymmetric piston bowl. First, injection and mixture preparation have been studied considering an early injection at the beginning of the compression stroke, and a delayed injection in the second half of the compression stroke. The results show how the higher in-cylinder pressure encountered by the delayed injection produces a jet characterized by a lower degree of under-expansion and a slower penetration. Moreover, the distribution of the in-cylinder mixture close to the ignition timing highlights that the stratification is greater for the late injection strategy. In both cases, the piston geometry also plays a crucial role in the mixture preparation because of non-conventional flow recirculation generated during the jet-piston interaction. Afterwards, combustion simulations have been carried out to further understand the effect of the injection timing on the premixed flame propagation. The results point out a reduced combustion duration for the delayed injection case. This can be explained by a twofold effect: a locally enriched mixture near the ignition point, speeding up the early flame kernel development, and a higher turbulence intensity around the ignition timing, which accelerates the overall flame speed.