Injection-timing-induced end-gas autoignition and knock control in a natural gas/diesel dual-fuel engine under lean operation

End-gas autoignition and the resulting rapid pressure oscillations are widely regarded as the primary mechanisms underlying knocking in internal combustion (IC) engines, which directly limits the thermal efficiency and operating load of natural gas engines. This study investigates the flame propagation and end-gas autoignition of a lean natural gas–air mixture (ϕ = 0.8) in a micro-pilot diesel–ignited dual-fuel engine using combustion visualization and in-cylinder pressure analysis in a compression–expansion machine. The effect of advancing the pilot diesel injection timing on auto-ignition and knock-limited operation was examined. The results show that progressive injection timing shifts combustion from normal operation to Premixed Mixture Ignition in the End-gas Region (PREMIER) and eventually to knocking with strong pressure oscillations. Both PREMIER and knocking exhibited a secondary peak in the cylinder pressure and heat release rate; however, autoignition occurred later in the PREMIER combustion phase, corresponding to a smaller end-gas mass and moderated volumetric heat release, limiting pressure-oscillation amplification. The knock intensity was markedly higher in knocking, with dominant pressure oscillation frequencies of 6.4, 10.0, and 14.5 kHz, compared with no dominant frequency for normal combustion. Flame propagation analysis revealed a faster growth of the auto-ignited flame area during knocking, with end-gas velocities increasing monotonically with knock intensity. Thermodynamic interpretation using Bradley’s ε–ξ framework provided mechanistic differentiation between the PREMIER and knocking regimes by mapping onto the CH₄–air detonation peninsula framework. The results suggest that controlled end-gas autoignition in PREMIER combustion can help extend the lean-burn operating range while maintaining the combustion stability and high efficiency of natural gas dual-fuel engines.