Ignition enhancement by repetitive pulsed discharge plasma in a methane/air mixture with barrier discharge igniter and J-hook plug

Methane, the main component of natural gas and biogas, is widely used in internal combustion engines as a low-carbon fuel. The stability of methane ignition under lean-burn and diluted conditions remains challenging for achieving high efficiency and low emissions. Repetitive pulsed discharge plasma offers a promising solution by enabling rapid energy deposition. In this study, a repetitive sub-microsecond pulsed discharge (RMPD) ignition system was developed and applied to a barrier discharge igniter (BDI) and a J-Hook plug. Their ignition characteristics were compared with conventional transistorised coil ignition (TCI) in a constant-volume combustion chamber. Methane/air mixtures with stoichiometric and lean conditions, λ = 1 and λ = 1.8, were tested at 2 bar and 298 K using high-speed Schlieren images and spectroscopic measurements. The lowest pulse number for successful ignition of RMPD systems was two, corresponding to the apparent discharge energy of approximately 6 mJ for BDI-RMPD and 32 mJ for J-Hook RMPD. For J-Hook TCI, the corresponding lowest tested energy was 45 mJ. BDI-RMPD exhibits the best ignition performance across all test conditions. However, J-Hook RMPD shows a strong dependence on pulse number, with “increase-decrease” non-monotonic behaviour at λ = 1 and improved ignition with increasing pulse number at λ = 1.8. Schlieren results indicate that the flame kernel with RMPD is synergistically governed by thermal effect and local plasma-induced jetting. Spectroscopic measurements of Hα (656 nm) and O (777 nm) emissions from the J-Hook RMPD show greater shot-to-shot variability under lean conditions, suggesting that the early ignition process is sensitive to local mixture conditions near the plug.