Endoscopic visualization of backfire behavior in a medium speed maritime hydrogen engine

Hydrogen is a promising energy carrier for decarbonizing maritime and stationary applications. However, using 100% hydrogen in large-bore engines introduces combustion challenges such as pre-ignition and backfire. These statistically occurring combustion anomalies, particularly their spatial and temporal behavior, cannot be fully understood through thermodynamic data alone. This study applies optical diagnostics to a medium-speed single-cylinder research engine (bore: 350 mm , stroke: 440 mm , displacement: 42 . 3 dm 3 ), operated with 100% hydrogen exceeding 20 bar IMEP. By varying the air–fuel equivalence ratio between 2.3 and 4.0, and comparing active pre-chamber and open combustion chamber ignition systems, backfire-induced operating limits are identified. High-speed flame imaging through two endoscopic accesses and up to three cameras captures both visible and UV ( 308 nm ) flame chemiluminescence. An implemented visual vibration compensation method using fiber optics enables tracking of flame origins and propagation. The recordings show that 65% of ignition events initiate near one intake valve, suggesting local hydrogen enrichment, confirmed via 3D-CFD simulations. This is linked to intake manifold geometry, which leads to mixture inhomogeneity up to − 26 0 ∘ CA BTDC . At loads above 15 bar IMEP , the localized enrichment reduces or shifts, attributed to increased turbulence and intake mass flow. CFD simulations also reveal that gas temperatures under the intake valves, exceeding the ignition temperature of hydrogen, as early as 30 0 ∘ CA BTDC , create the risk of backfire in the early gas phase. Additionally, glowing oil droplets and ignition zones near the piston were observed, indicating that lube oil ignition may be a cause of later (after − 29 0 ∘ CA BTDC ) backfire events. These findings contribute to the understanding of hydrogen combustion anomalies and support future experimental and modeling-based optimization of large-bore hydrogen engines. • Minimal invasive optical insights of backfire behavior in a medium speed 42.3 L single cylinder engine running with 100% hydrogen. • Optical vibration compensation method for large bore engines using modified endoscope sleeves with included optical fibers. • Wide load and air–fuel-ratio variations of 8 – 20 bar IMEP and AFER = 2 . 3 – 4 . 0 , respectively. • Determination of backfire origin distribution due to inhomogeneities in mixture formation in the early gas phase and load levels. • Probable backfire initiation due to hot residual gas and reactive lube oil.