Knock tendency in ultra-lean hydrogen combustion using an active pre-chamber fueling strategy

Hydrogen-fueled spark-ignition engines enable ultra-lean combustion. However, combustion instability and knock phenomena remain major limitations under highly diluted operating conditions. The phenomena were studied in an engine equipped with a spark-jet ignition system that enables extended combustion control via an additional direct fuel injection into the pre-chamber. The study was conducted on an AVL 5804 single-cylinder engine, adapted for hydrogen combustion. Under conditions of variable air excess ratio from λ = 1.6 to 3.0, the effect of changing the PC injection duration from 0 to 10 ms on thermodynamic parameters, knock intensity, and exhaust emissions was evaluated. In-cylinder processes were identified by simultaneously recording combustion pressure in the cylinder and the pre-chamber. Knock combustion behavior was analyzed based on the MAPO index (maximum amplitude of pressure oscillations). According to the research, an active pre-chamber operation enables stable ultra-lean combustion (λ up to 3.0), which is not achievable with a passive configuration. A passive pre-chamber (PC) with lean conditions leads to combustion deterioration and partial misfire. Regulating the amount of fuel supplied directly to the PC significantly affects combustion phasing and extends the ultra-lean operating range. Knock may still persist locally within the pre-chamber. Conducting combustion under ultra-lean conditions allows for a reduction in NO x emissions while maintaining combustion efficiency above 99.6% across the investigated operating range. The study provides new insight into the influence of pre-chamber fueling on combustion phasing and knock-related pressure oscillations under ultra-lean hydrogen combustion conditions.