Investigations of combustion characteristics and mechanism of backfire-induced super-knock in a turbocharged hydrogen engine

Hydrogen internal combustion engines have been demonstrated to offer high power with zero carbon emissions. However, knock pose a significant obstacle to enhancing hydrogen engine performance further. Super knock commonly occurs in highly boosted engines with low-speed pre-ignition regimes while a critical super-knock induced by backfire with a new mechanism is detected in hydrogen engines. Experiments and multi-cycle numerical simulations are conducted on a 5.13 L port fuel injection turbocharged hydrogen engine. The results indicate that backfire leads to a maximum pressure amplitude oscillation of 134.7 bar super-knock in the next cycle, which is attributed by twice auto-ignition at −1.3/3.6 °CA. Backfire rises the local temperature to 915 K at the bottom of the cylinder and scrambled hydrogen distribution, leading to large locally rich regions of lambda = 1.2, a lean mixture of lambda = 2.9 near the spark plug, results in 5.4 × 10⁻⁴/2 × 10⁻³ maximum mass fraction of HO₂/H₂O₂. The mechanism of super-knock induced by backfire and normal knock caused by advanced ignition are compared, the unburned region size is positively correlated with the knock intensity. This study of super-knock induced by backfire mechanism is first proposed and it reveals insights of avoiding super-knock, thereby achieving higher performance hydrogen engines.