Study on injection-induced multi-scale flow in a hydrogen-enriched natural gas direct-injection engine using dynamic mode decomposition

Unlike port fuel injection engines, direct-injection engines deliver high-speed fuel jets directly into the cylinder, strongly shaping in-cylinder flow, mixture formation and combustion. This study investigated a hydrogen-enriched natural gas (HCNG) direct-injection engine using large eddy simulation (LES) at 2000 r/min with 30 vol% hydrogen. Dynamic mode decomposition (DMD) was adopted to compare two dominant mode screening methods, and the modes were categorized by spatial scale with frequency as an auxiliary indicator. The interaction between the fuel jets and pre-existing in-cylinder flow was analyzed from a multi-scale perspective. The results show that the vorticity exhibits a three-peak evolution. The first peak at −300 °CA ATDC is driven by the intake jet and tumble flow, decaying to 11% before injection. The second peak at −110 °CA ATDC is induced by fuel jets shear, while the third peak at −15 °CA ATDC is caused by late-compression squish flow. DMD results show the global energy method outperforms the amplitude method. The mode order is generally negatively correlated with spatial scale. Spatially, under the injection condition, Mode 1 is the mean flow. Modes 2–5 show part of the large-scale counterclockwise circulation. Meso-scale Modes 6–7 present an injection-induced double-vortex structure. Small-scale modes have small differences. Energetically, high-energy-fraction Mode 1, Modes 4–5 and 6–7 deserve special attention. Large-scale modes have the highest energy fraction, followed by small-scale modes at 30–40% and meso-scale modes at 10–15%. These findings evaluate flow structure importance and may affect mixture formation, providing a potential basis for combustion optimization. The LES-DMD framework may transfer to other gaseous-fuel direct-injection systems.