Effect of injection timing on gas jet developments in a hydrogen low-pressure direct-injection spark-ignition engine

Injection timing in low-pressure hydrogen direct injection (H 2 LPDI) engines plays a critical role in optimising gas jet structure and mixture formation due to the complex and transient nature of ambient air flow and density inside the cylinder. This study systematically investigates the macroscopic characteristics of gas jet development at five distinct injection timings from 210 to 120 °CA bTDC with the intake valve closure (IVC) as a reference point in a motored, inline four-cylinder spark-ignition engine at 2000 rpm and 160 Nm load using low-pressure injection of 3.5 MPa. Optical access was made with two endoscopes: one for high-speed imaging and the other for laser insertion to realise laser shadowgraph imaging of the gas jet delivered using a side-mounted, outward-opening pintle nozzle injector. The experimental results reveal spatial and temporal variations in jet morphology, penetration, spreading angle and mixture dispersion as a function of injection timing. Pre-IVC injection (210 °CA bTDC) produced a narrow mean cone angle of ∼40° and the highest penetration-rate proxy (0.49), whereas post-IVC injection (120 °CA bTDC) retained a wider ∼53° cone yet reduced the penetration rate to 0.28 while increasing the sheet-based mixing index from −0.084 to −0.106. Pre-IVC injection, occurring under low ambient pressure and with active intake airflow, was found to produce elongated jets with enhanced penetration and mixing rates, though accompanied by substantial cyclic variations. Conversely, post-IVC injection was strongly influenced by a fully developed tumble flow, which redirected the jet trajectory towards the pent-roof and facilitated mixing through increased turbulence. However, the elevated air density constrained the jet penetration. At-IVC injection resulted in a more uniform and stable jet structure. However, the lack of convective flow constrained the overall mixing effectiveness. Quantitative analysis of jet spreading angle, pixel intensity gradient and centroid movement using 100 consecutive cycles confirms the critical role of injection timing in shaping the gas jet development as suggested by the images. • Endoscopic gas jet imaging achieved for varied flow field conditions in a real hydrogen engine. • Jet morphology and mixing vary significantly with injection before or after IVC. • Pre-IVC injection enhanced penetration but showed unstable jet boundary. • Post-IVC injection limited penetration but improved jet mixing via tumble interaction. • Injection at IVC yields symmetric jets with reduced mixing effectiveness.