Numerical simulation and experimental validation of NO emissions in a heavy-duty H$_2$DI engine considering injector needle dynamics and multi-cycle analysis

Hydrogen direct-injection engines offer a promising pathway for decarbonizing heavy-duty transportation, but accurate
prediction of mixture formation and NO$_x$ emissions remains challenging due to complex injector dynamics and strong
cycle-to-cycle variability. This work presents a comprehensive computational and experimental investigation of supersonic
H$_2$ direct injection, mixing, combustion, and NO formation in a single-cylinder heavy-duty hydrogen engine operated at
1100 rpm and λ = 2.6. A detailed three-dimensional CFD model is developed, coupling a pressure-based injection bound-
ary condition with a realistic Bosch F2 prototype injector needle-lift profile to capture valve-bounce effects. The model
is validated against measured in-cylinder pressure, fuel and air mass, and NO emission data. Multi-cycle combustion
behavior and NO emission variability are analyzed using the concurrent perturbation method (CPM), with 20 statistically
independent realizations at reduced computational cost. Results show that near-spark mixtures with higher fuel concentra-
tion accelerate flame propagation and increase peak NO by a factor of two (76 ppm vs. ... mehr32 ppm). Simulations reveal that
NO forms predominantly in local pockets of high fuel concentration, with turbulent flame speeds of 11–22 m/s during the
early combustion phase. Predicted exhaust-port NO levels agree qualitatively with experiments, though unsteady RANS
tends to overpredict NO due to limited small-scale mixing. The study demonstrates that resolving the injector flow rather
than approximating boundary conditions, combined with CPM can effectively capture hydrogen combustion dynamics
and emission variability.