Amid　growing　environmental　concerns,　hydrogen　(H2)　is　emerging　as　a　prospective　alternative　fuel　for　driving　internal　combustion　engines.　Employing　lean　combustion　technology　in　tandem　with　turbulent　jet　ignition　(TJI)　has　the　potential　to　enhance　combustion　rates　while　mitigating　NOx　emissions.　Therefore,　an　experiment　was　developed　to　investigate　the　combustion　characteristics　of　ultra-lean　premixed　H2/air　by　TJI.　An　active　pre-chamber　(PC)　with　an　additional　H2　supply　was　selected.　Moreover,　the　effect　of　nozzle　structures　and　equivalence　ratio　was　discussed.　The　results　show　that　with　a　nozzle　diameter　of　3　mm　and　an　elevation　of　ΦPC　to　1.4,　the　lean　flammability　limit　is　extended　to　an　equivalence　ratio　of　0.13,　with　a　consistently　stabilized　ignition　delay　within　4　ms.　Increasing　the　nozzle　number　also　extends　the　lean　flammability　limit,　but　it　incurs　higher　energy　losses.　Additionally,　two　ignition　mechanisms　exist　in　TJI:　flame　ignition　and　combined　ignition.　The　transition　from　flame　ignition　to　combined　ignition　commonly　occurs　when　the　equivalence　ratio　of　the　main　chamber　drops　below　0.3.　This　transition　typically　results　in　higher　peak　pressures　and　burnt　fuel　ratio,　lower　combustion　duration,　and　longer　ignition　delay.　©　2024　Elsevier　Ltd