This study investigates edge-flame propagation under hydrogen–diesel dual-fuel direct injection (H₂DDI) engine conditions using two-dimensional direct numerical simulations. A single-fuel baseline case (DF00) and two dual-fuel cases (DF01 and DF05) with increasing hydrogen addition are examined. A local Damköhler-number-based indicator is employed to interpret the relative roles of reaction- and transport-driven processes. A statistical framework is developed to quantify the edge-flame speed and its dependence on local scalar dissipation. The results show that ignition behaviour varies systematically with mixture stratification. In cases DF00 and DF01, the edge-flame speed exhibits a non-monotonic dependence on scalar dissipation, accompanied by the presence of both autoignition-like and deflagration-like behaviours across different dissipation levels. With increased hydrogen addition, the edge-flame speed increases monotonically with scalar dissipation and attains higher most-probable values, with a greater prevalence of deflagration-like behaviours.

Novelty and significance statement: To date, the characterisation of edge-flame dynamics in hydrogen–diesel dual-fuel direct injection systems remains limited. This work combines temporal tracking and statistical analysis of edge flame intersection dynamics to elucidate how hydrogen addition modifies edge-flame propagation under engine-relevant conditions. The results demonstrate that hydrogen reshapes the dependence of edge-flame speed on scalar dissipation and alters the dominant ignition mode. A Damköhler-number-based indicator is employed as a physically grounded measure to distinguish autoignition-like and deflagration-like behaviours. These findings clarify how hydrogen modifies edge-flame dynamics in stratified dual-fuel ignition and provide physical insight for combustion modelling and ignition control under engine-relevant conditions.